Police have been called to the University of Sydney after a rally encouraging students to vote "no" in the same-sex marriage postal survey drew hundreds of counter-protesters.

About 200 students gathered around a few students who were making speeches and holding signs that read "It's ok to say 'no.'"

The confrontation occurred shortly after midday on Thursday near a stall that is believed to have been set up by the university's student-run Catholic society.

Kishor Napier-Raman, 21, who is a law student at the university and an editor of the student newspaper Honi Soit, said, "The Catholic society tried to have a 'vote no' rally with the promise of free kebabs, but it seemed to have been quickly surrounded by a much bigger counter-rally.