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TMZ has obtained EXPLOSIVE VIDEO of 2 Chainz and his crew being robbed at gunpoint in San Francisco this weekend ... and the footage shows the chaotic moments as the rapper and his crew desperately ran for their lives.

2 Chainz and his entourage were casually walking down the street Sunday afternoon when at least 2 men approached the crew and pointed guns at them ... one in a white shirt, the other in a black hoodie.

2 Chainz and his crew immediately scattered -- frantically trying to sprint down the street while the gunmen chased after them.

One of the gunmen seemed to be having fun during the incident ... and gleefully skipped after the rapper while 2 Chainz tried to get away.

During the chase, 2 Chainz stumbles and falls ... and one of the gunmen pounces ... hovering over the rapper with his gun drawn.

According to police, the bad guys jacked 2 Chainz for his cell phone and wallet. Towards the end of the video, you can see a man running back up the street with what appears to be the loot stuffed under his clothes.

Under the category: "Who ya gonna believe, me or your lyin' eyes" ... 2 Chainz later posted a tweet denying he was robbed ... saying, "Rule #1 if a rapper gets robbed people usually post items that has been taken."

Miley Cyrus says it's "impossible" that she's more than friends with Justin Bieber ... despite the public flirting ... 'cause she's currently engaged to someone else.

Miley called in to Hot 99.5 in D.C. this morning ... and actually responded when the DJ asked if the photos on TMZ showing Miley and Bieber flirting in public this weekend prove they're "more than friends."

Her response: "I'm ENGAGED!" she said, chuckling. "I'm engaged. That is impossible."

It's an interesting statement for two reasons -- first, it clears up the rumors that she broke things off with Liam Hemsworth.

Second, when it came to being more than friends with Justin ... her answer isn't exactly a strong "no."

Miley was also asked about Amanda Bynes -- and she says some interesting stuff.

Give it a listen.

Kim Kardashian's bff Jonathan Cheban -- a regular on "Keeping Up With the Kardashians" -- found himself in the middle of an airport emergency when a woman on his plane threatened to stab him with a knife.

Cheban was flying from JFK to LAX -- when a woman on his flight tweeted at him, "Jonathan Cheban is on my flight, should I cut him with my weapon?"

She added in another tweet, "I gotta pretty intense pocket knife with mehhh."

Cheban, a former PR exec, responded, "Police?"

When the plane landed at LAX, the woman was escorted off by police and subsequently detained. We're told cops were deeply concerned about her claim that she got a knife through airport security.

She later tried to bury the hatchet, tweeting back at Cheban, "It was a bad joke ... you look really hot babe." But airport police weren't taking any chances.

Police searched the woman outside the plane and determined she did NOT actually have a knife. She was subsequently released without further incident.

Cheban tells TMZ, he never personally alerted authorities -- someone else reported the tweets -- but later in Bev Hills he admitted the incident was very "scary."

2 Chainz did NOT get robbed at gunpoint, but he does give San Francisco marijuana really, really high grades ... this according to the rapper himself.

As we previously reported ... San Francisco PD says 2 Chainz got jacked yesterday when someone fired shots in a crowd that included the rapper and his entourage.

But 2 Chainz sounds like he's denying anything went down at all -- because he tweeted, "Rule #1 if a rapper gets robbed people usually post items that has been taken. Rings, chains, watch, money etc."

He also says, "Rule #2 if a rapper gets shot he usually go to hospital or dies."

Police told us 2 Chainz said he wanted to handle the matter himself ... so the denial seems to back that up.

Andy Richter says "two drunks" slammed into his car last night while it was parked in front of his L.A. home ... and the funnyman says it happens so frequently, he's MOVING.

The "Conan" sidekick posted a photo of the wreckage on Twitter -- writing, "Two drunks totalled my car while it was parked in front of my house tonight."

"We live on a busy street. We have for 11 years. We grew tired of this, and bought a house on a quiet street."

He continued, "Our old house goes on the market in just a few days. We are moving in three weeks."

Richter referred to last night's collision as a "kiss goodbye."

We've reached out to cops and Richter for more details ... no word back.

Formerly troubled "Terminator 3" star Nick Stahl has some invaluable words of advice for Amanda Bynes ... if she wants to turn her life around, she should start by pulling the plug on her crazy Twitter account.

Stahl -- who famously struggled with drugs, alcohol and other personal issues in the last year -- tells TMZ Amanda's habit of constantly chronicling her every waking move isn't healthy ... and she should spend her time reconnecting with friends and family instead.

But Stahl's still optimistic, adding, "She'll figure it out. She's young."

Ferraris apparently aren't fast enough for Justin Bieber .... 'cause the singer has just been booked on a ROCKETSHIP DESTINED FOR SPACE ... so says Virgin Galactic honcho Richard Branson.

Branson himself confirmed that Biebs and manager Scooter Braun are on the V.G. manifest, tweeting this morning: "Great to hear @justinbieber & @scooterbraun are latest @virgingalactic future astronauts. Congrats, see you up there!"

Branson's venture just completed its first rocket-powered test last month, and the company is planning to start commercial flights next year -- plenty of time for Bieber to order a tacky, blinged-out astronaut suit.

Tickets reportedly start around $250k ... but it's unclear if JB is paying for the ride himself ... or if the trip will be comped.

No word on when Bieber is scheduled for lift off.

Tyler, the Creator says Keyshawn Johnson is DEAD WRONG -- claiming HE was driving Justin Bieber's Ferrari during the infamous chase through Calabasas.

Tyler tweeted the admission this weekend ... writing, "That Was Me Behind The Wheel Of Justins Vehicle."

He added, "Don't Blame Him. People Are Actually Evil Swearing Up And Down That It Was Who It Wasnt."

Bieber tweeted a response -- "All good buddy. i deal with this everyday. thanks though. the baby aint mine either."

Tyler's message was clearly intended for Johnson -- who told TMZ he personally witnessed Justin in the driver's seat after the car dangerously sped through their gated community. We're told at least 3 other people also saw Justin behind the wheel.

Either way, the residents of Justin's neighborhood are PISSED -- and are threatening to withhold their HOA dues until the community takes action against Bieber.

Amanda Bynes followed through on her promise to get her nose fixed after seeing it in her mug shot ... and she tells TMZ she already wants to get another procedure ... ASAP.

In one of her post-arrest Twitter rants, Bynes said she was planning on "getting in shape and getting a nose job!"

Bynes -- who previously copped to having a nose job weeks before her arrest -- tells TMZ she got the procedure done yesterday in Manhattan and explains, "I have no bandage on. It's healing on it's own like my doctor asked."

But she's not stopping there, telling us, "I'm getting one more in 3 weeks, they are short amazing surgeries done while I'm awake but under general anesthesia. It's almost perfect."

She says once her nose "doesn't look so awful in photos" she will stop hiding from the paps.

He's baaaaaaaaack!!!

Dan Harmon, the creator of "Community" who was unceremoniously dumped from the show last year, is finalizing his deal to return to the show, so says The Hollywood Reporter. THR says the deal should be closed in the coming days.

Harmon's return has been rumored for the last week, and he tweeted earlier today, "Yes yes yes! I'm back I'm back I'm back. You can thank @joelmchale" in response to a fan's question.

He also tweeted earlier several times about the gloriousness of a certain airport bar, so take that for what it's worth.

Ashley Tisdale's obsessed fan just could not stay away from the star ... on social media, at least ... and was arrested yesterday for violating a restraining order, TMZ has learned.

As TMZ first reported, Tisdale was granted a TRO against Nicholas Fiore ... claiming she feared for her life after Fiore threatened to shoot her and her BF, and hammered Tisdale with more than 18,000 tweets since 2012.

Law enforcement sources tell TMZ that Fiore violated the restraining order more than once since it was put in place. We're told the alleged violation was not for personal contact, but rather contact via social media.

The person in charge of the President's Twitter account posted this pic today of Barack Obama admiring something on Justin Timberlake's phone.

While they both look spiffy in their suit and ties, we gotta ask ...

Despite all the craziness ... and the wigs ... and the legal drama, David Spade says he still sees Amanda Bynes for who she really is -- a hot piece of ass.

Spade was hangin' at The Improv last night with his "Grown Ups 2" co-star Nick Swardson -- when Dave dropped some serious truth bombs about the ongoing Bynes saga.

"I'm scared of Amanda Bynes," Spade said ... joking that she'll call him "ugly" on Twitter if he says anything unflattering about her.

But Spade did rank Amanda's hotness -- and it's obvious, he's a fan.

"If you cut out all the 'stuff,' ... she's technically, for all intents and purposes, very pretty -- but I think she's getting in her own way."

Amanda Bynes is now claiming the tweets bashing Rihanna (pardon the pun) did not come from her and were actually mocked up by someone else ... then she went on to say some other crazy stuff.

Bynes went on another rambling Twitter run this afternoon, in which she says, "I saw a bunch of mocked up tweets about me bashing Rihanna in my mentions. I'm followed by so many people that someone is always mocking up fake tweets so I feel the need to address them!"

From there she goes on to say ... well, your guess is as good as ours:

"That's one of the mocked up images, they took photos of me from outside and morphed them onto someone else's body. I am allergic to marijuana and alcohol but I smoke tobacco. Why does Rihanna smoke weed and not get in trouble for it but I smoke tobacco and people think I'm on drugs? I refuse to be treated like someone I'm not, which is why I fought for myself and am suing everyone involved. I don't need to go to rehab. There is never a drug or alcohol in my system! I'm sick of all the lies! What would you do if someone accused you of things you didn't do and yet you still had to be in jail at all over it! I'm so offended but I am so educated that I know cops cannot illegally enter my apartment, sexually harass me, arrest me, take me to a MENTAL HOSPITAL, then lock me up for a crime I didn't commit. I'm suing them all for this upsetting nightmare. My lawyer knows I'm a model citizen who doesn't partake in drugs. He's going to court this week to set the record straight again on my behalf. Thankfully I'm an educated multi-millionaire who knows better than to speak to perverted unjust cops without my lawyer."

Amanda Bynes says she is planning to unleash a hailstorm of lawsuits against the NYPD and her apartment complex ... all over her arrest earlier last week ... or as she puts it, her "mistaken arrest."

Bynes just went on another glorious Twitter rampage, saying she plans to file a lawsuit against the NYPD for:

-- "Illegally entering my apartment"
-- "Lying about drugs on me"
-- "Lying about me tampering with non existent drug paraphernalia"
-- "Being put into a mental hospital against my will"
-- "Then locked up overnight for coming home after a facial and working out with my trainer like the good girl that I am."

Bynes also claims she's allergic to drugs and alcohol and is "proud to not be a drug or alcohol user."

Amanda Bynes just inexplicably railed on Rihanna on Twitter, saying Chris Brown beat her because she's ugly ... but RiRi got the last word and it was AWESOME.

Bynes tweeted -- and quickly deleted -- "@rihanna Chris Brown beat you because you're not pretty enough."

She followed that up with another insult -- about wanting to name her dog Rihanna. It's unclear what set her off.

RiRi clearly got wind of the deleted tweets, because she just posted the BEST COMEBACK EVER:

Don't mess with RiRi.

UPDATE 11:03 AM PT -- Amanda has already responded (and already deleted):