"The city police have arrested 15 people so far. I don't have details of where they were arrested. We're still processing them," Ruth Miller, a spokeswoman for a city information centre set up for the G20 summit, told AFP.

Riot officers intervened when a 1,000-strong crowd, led by black-clad hardliners wearing helmets and masks and brandishing anti-capitalist banners, began to trek across Pittsburgh towards the conference venue.

The Group of 20 is a forum for the world's biggest developed and emerging economies and its meetings are a magnet for anti-capitalists opposed to what they see as an undemocratic body promoting inhumane free market policies.

On Wednesday, 14 people from pressure group Greenpeace were arrested after they unfurled a huge banner from one of Pittsburgh's bridges, with a message to the world leaders attending the summit to act to stop global warming.

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