University student services director David Richardson said the university was aware gang affiliates and drug dealers had been attracted to the "4:20" protests, and it was "very concerned".
The concerns had been raised with police, who had been asked to respond.
[comment caption=Should the police crack down on the student smokers?]However, the leader of the Otago branch of marijuana law reform group Norml, which has held protests on the campus for four years, said the majority of protesters were students, and non-students were asked to leave.
Dunedin area commander Inspector Dave Campbell said the activities of the group, which held protests on Wednesdays and Fridays, had to stop.
"They are certainly not welcome on the campus if they are going to flout the law," Insp Campbell said.
"At the moment, they are thumbing their noses at the authorities."
Norml is aligned to the Otago University Student's Association Clubs and Societies group, and this year it has attracted up to 100 people to its twice-weekly protests.
The university has consistently said it does not condone illegal activities of any kind.
Mr Richardson said the university had noticed an increase in numbers at the protests.
Insp Campbell said police had monitored the group for several months, and was concerned "known gang members" were associating with the group, along with known cannabis dealers and people with drug dealing convictions.
Secondary school pupils had also been seen with the group.
Police estimated about one-third of the people at the protests were not students.
Otago Norml spokesman Abe Gray confirmed gang members and underage people had tried to join the group, but he said they were turned away.
A group of drunk people had also been turned away from a protest, he said.
Insp Campbell said police would work to remove the group from the campus by issuing trespass notices to non-students. This had been authorised by proctor Simon Thompson.
Students seen smoking cannabis would be referred to the proctor.
A small number of trespass notices had already been issued to non-students. The notices were issued off campus.
Insp Campbell said it was difficult to gather enough evidence to prosecute the group on cannabis charges.
If the group continued to congregate, police would continue to issue trespass notices.
"Put it this way - we have enough photocopy paper to continue to make trespass notices for a long time yet."
Insp Campbell said trespass notices would be issued "on the spot", and he hoped the students would have enough sense to "move on".
University staff believed the "vast majority" of students did not want the group on their campus.
Mr Gray said the group was sick of threats from the police, who had failed to put any of their words into actions.
He believed they were using scare tactics. The group was acting on what it believed was a public mandate for the law on cannabis to be changed, he said.
