Riot police in Pittsburgh, in the eastern U.S. state of Pennsylvania fired teargas and pepper spray to break up what it says was an unauthorized protest against the Group of 20 economic summit.
Many of the marchers wore black and carried signs denouncing capitalism. They tried to head from from a city neighborhood to the convention center, where world leaders are gathering.
Police warned the demonstrators they did not have a permit to gather and urged them to leave. Officers fired tear gas and set off high-pitch sirens to force the marchers to break up. No injuries or arrests have been reported.
Meanwhile, the White House says the top priority for the G20 is regulating financial markets that let risky investments plunge the world into a deep recession.
But the G20 leaders are torn on the best way to tackle this challenge. They also disagree on how much control the international community should have over financial markets.