Organisers from the Extinction Rebellion movement had vowed to close Princes Bridge near Flinders Street Station on Saturday to raise awareness of the "climate crisis".

More than 300 people activists flocked to the bridge from midday and were gone about three hours later.

"The seriousness of the global climate crisis demands mass non-violent, disruptive action to get the kind of urgent change we need if we want to avoid catastrophe," spokeswoman Miriam Robinson said.

"We know this type of action is inconvenient for some and we don't set out to disrupt people's lives needlessly."

Victoria Police had formally warned protesters twice that if they did not move they would be arrested, before public order officers started to arrest people.

"A number of protesters complied but police had to arrest 36 protesters who refused to leave the roadway," a police statement read after the protest.

"They are expected to be charged on summons for obstructing a roadway."

One woman told AAP she received a summons for "sitting on a bridge".

Police including mounted officers blocked both sides of the bridge and stopped members of the public crossing during the protest.

People had a right to protest peacefully without impacting the rest of the community, a police spokeswoman said.

Australian Associated Press