Pittsburgh, Sep 25 (DPA) Police in Pittsburgh, Pennsylvania Thursday used tear gas to disperse dozens of demonstrators gathered to protest the Group of 20 summit, media reports said.

Nearly 500 marchers, carrying anti-capitalist signs and wearing bandanas and gas masks, were warned in English and Spanish by police that they were not authorised to march into a certain area, the Pittsburgh Post-Gazette newspaper reported.

Their path was blocked several times by riot police, dispersing part of the crowd and eventually the police fired tear gas on the remaining protestors.

Leaders from the world's 20 leading economies are meeting in the city through Friday to hammer out reforms in the wake of the global recession.

More than 4,000 police have been called into service during the summit. Dozens of groups supporting various causes, including Tibet and the environment, planned to stage protests around the city of 300,000.

About a dozen Greenpeace activists were arrested after rappelling off a bridge and unfurling a banner, the paper said.