Justin Bieber 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 ...