Several officers were on duty outside the Grand Hotel in Stockholm on Wednesday to help control frenzied Beliebers who had gathered outside the hotel in hopes of catching a glimpse of their music idol.
But when the 19-year-old heartthrob stepped onboard, one of the officers became suspicious that drugs were being used on board.
"A colleague sensed what was considered to be a strong smell of marijuana," Stockholm police spokesman Lars Byström told the Aftonbladet newspaper.
Officers subsequently raided Bieber's tour bus after it had arrived at Stockholm's Globe Arena shortly before the pop music sensation was set to take the stage.
According to Byström, officers confiscated a "small amount" of what was believed to be marijuana.
"A stun gun requiring a permit was also found on the bus," he added.
The suspected drugs have since been sent to a lab for testing, but so far no one has been suspected of any crimes.
"We don't have any concrete suspicions against any specific person; there were several people on the bus," Byström told Aftonbladet.
Bieber made headlines earlier this year when he was photographed smoking a joint.
Before coming to Stockholm, Bieber played to sold-out venues in Norway and Denmark. In Sweden he surprised fans with an impromptu dance performance outside his hotel.
But the teen idol's European tour has been marred by controversy.
Earlier this month, Amsterdam's Anne Frank museum defended Bieber amid a furore over his comments that he hoped the Jewish teen who died in the Bergen-Belsen concentration camp would have been a fan.
And in Germany, animal rights activists asked him to apologize for bringing a 14-week-old capuchin monkey into the country without the proper paperwork.