Thirty-six environmental activists have been arrested after they disrupted traffic until police physically carried them off a major bridge in Melbourne's CBD.

About 200 people took part in the Extinction Rebellion rally, holding signs with messages such as "no prosperity on a ruined planet" and "declare a climate emergency".

The protesters blocked tram services and traffic on Princes Bridge for several hours.

Scores of police, including some on horseback, issued verbal warning to the protesters before carrying them from the scene one-by-one and placing them under arrest.

"Protestors assembled on the bridge and were asked by police to move off the roadway," a police statement said.

"A number of protestors complied but police had to arrest 36 protestors who refused to leave the roadway.

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

The rally was staged by the Extinction Rebellion movement. ( AAP: James Ross )

Ahead of the rally, a statement from organisers said activists were "ready and willing to be arrested" in their bid to draw attention to the "long-standing negligence of our governments in facing up to the unfolding climate and ecological catastrophe".

It said the rally was part of a series of "large and disruptive" protests in Australia and internationally.

Princes Bridge is a major thoroughfare at the end of Swanston Street, near Flinders Street Station and Federation Square.