Britney Spears' 'Ruff' Dinner Turns Heads at L.A. Restaurant

8:21 AM PT -- A rep for Britney tells TMZ ... "This is completely blown out of proportion. Britney was enjoying a quiet dinner with her assistant and bodyguard. She was simply telling the story about how her dog was barking at the neighbors."

They continue, "At no point did she put anyone in danger with a knife. She was cutting her hamburger in half. This constant attack on everything that she does and this is exactly what happened 20 years ago when the media tried to depict Britney as a bad person. This is ridiculous and it needs to stop now."

Britney Spears enjoyed a night out with friends at a Los Angeles area restaurant ... but TMZ has learned things got erratic fast -- with barking, yelling and even a knife involved.

Here's the deal ... sources tell us the Princess of Pop grabbed a corner table with a man and a woman at Blue Dog Tavern in Sherman Oaks Wednesday night ... and quickly started causing a scene.

We're told Britney was raising her voice, screaming, and even barking at times ... with witnesses describing the vibe as chaotic and "kind of sad" ... though she still looked cute.

At one point, a restaurant patron tells us Britney walked by their table holding a knife, sparking fear she might accidently stab someone. The witness says they believe Britney may have simply forgotten to put it down at her table.

We're also told the singer lit a cigarette inside near the door ... prompting staff to intervene and ask her companion to have her put it out.

As for food, we're told Britney ordered a burger and fries but barely ate, mostly picking at the fries. Plenty of food was left behind, along with what appeared to be orange juice.

We're told Britney and the man she was with were feeding each other and she told him, "I love you" ... though it's unclear whether she meant it or was just being playful.

The aftermath wasn't pretty either ... one guest said the area under her table looked "like a toddler had been there."

TMZ obtained a photo from the table after Britney left ... showing a crumpled, folded-up menu left behind amid the messy scene.

Despite all the commotion, we're told most diners didn't even realize it was Britney until after she left -- at which point it became the talk of the restaurant, with people saying it was something that would "only happen in L.A."

Our sources say Britney was taken home by her security.

As you know, it was less than 2 weeks ago Britney got her March DUI charge dropped after reaching a plea deal with prosecutors, which allowed the charge to be reduced to a "wet reckless." In her deal, she agreed to be on probation for 12 months and only possess drugs she is prescribed.

She's also ordered to continue with mental health and substance abuse treatment, including meeting with a psychologist once a week and a psychiatrist twice a month.

We broke the news -- the pop superstar went to rehab at Borden Cottage in Camden, Maine, a rural facility that provided her with substance abuse and mental health services following her DUI arrest.