TMZ has spoken with a rep for the Stockholm PD, who told us ... Around 7:10 PM on Wednesday night, an officer smelled weed coming from Justin's bus, which was parked in front of the Grand Hotel.
Cops say ... when the bus left the hotel and headed toward the Globe Arena, the officer contacted a special narcotics unit, which issued a warrant to search Bieber's bus.
We're told cops searched the bus in the parking garage of the Globe Arena -- no one was on the bus at the time.
Cops say they found a small amount of narcotics (they won't say which drug, but local papers say it's weed) -- along with a taser.
We're told cops have not identified a suspect yet ... and the bus has not been impounded.
Bieber had been seen with his pal Lil Za in the hours before the weed incident ... and FYI, Za is the guy who was seen smoking pot with Bieber at an L.A. home earlier this year.
Cops tell us NO ONE WILL BE CHARGED because they have no idea who was in possession of the drug since the bus was empty. They found the narcotic on the floor of the bus so they have no clue who brought it on.
is so rock n' roll -- his tour bus was raided by Swedish police who allegedly found marijuana on board Wednesday night ... according to a Swedish newspaper.
Police reportedly boarded the bus in Stockholm as the bus pulled up at the arena where Bieber was performing ... and found a "small amount" of what appeared to be pot.
A police spokesman told the Aftonbladet newspaper ... they first detected a "strong smell" of marijuana coming from the bus as they were controlling the huge crowd that had gathered outside Bieber's hotel.
The Swedish report says police also found a stun gun ... which requires a permit.
No arrests were made ... and it's unclear if Justin was on the bus, or if he'd already entered the arena, when the raid went down.Story developing ...