Montreal Mayor Gerald Tremblay has issued a statement saying Occupy protesters at Victoria Square must leave.
"The time has come for the protesters to find another way to get their message out," Tremblay said in his statement.
He said peace and security are not being maintained on the site.
The mayor's comments come after media reports about fights on the site on the weekend.
"I am sure the protesters will understand the need to act as quickly as possible," Tremblay said.
Some protesters at the site told CBC News they will not leave willingly.
"We're going to keep on fighting the class war," said Chris Brown. "I'm not going to leave until I'm actually forced to leave. I don't plan on leaving until the last second. I'm going to keep on fighting."
The mayor has not given a formal deadline for the protesters to vacate the site.
Up until now, Tremblay had said the protesters could stay at Victoria Square, provided they continued to co-operate with police, firefighters, and the city.
Speaking with reporters Monday, Tremblay said the protesters told the city they could no longer guarantee the site is safe.
CORRECTION: A previous version of this story erroneously referred to Montreal's mayor as Gordon Tremblay.