Violent clashes erupted Saturday between police and rock- and bottle-throwing demonstrators outside a meeting in Australia of some the world's top financial officials.
No injuries were immediately reported, as skirmishes continued.
Police on horseback and other plainclothes and riot officers were succeeding in keeping protesters out of the plush hotel where U.S. Federal Reserve Chairman Ben Bernanke and top officials from Europe, Asia and Latin America opened two days of talks on global economic issues.
But running skirmishes occurred in the streets nearby, where protesters ripped apart traffic barricades and sprayed slogans on at least one building, Associated Press reporters and photographers at the scene said.
At one place, about 200 demonstrators rained stones, glass bottles and plastic garbage bins down on about one dozen police standing near a police van and truck parked outside the security perimeter, an AP photographer said.
The police ducked behind the vehicles to avoid the barrage, until a contingent of mounted police charged from behind the security fence at the demonstrators, who ran off.
An official police media spokesman declined to say if there were any reports of injuries or arrests, saying the situation was ongoing.
About 3,000 people rallied at a city park around midday Saturday, then marched on the meeting of the Group of 20 finance minister and central bankers. But most of the violence appeared to center around a group of about 200 demonstrators dressed in white coveralls with red bandanas tied around their faces.
The group ran from one location to another near the venue, challenging police before retreating.
The unrest recalled the widespread violence at anti-globalization protests that marred the World Trade Organization's meeting in Seattle in 1999, and a meeting of the World Economic Forum in Melbourne the following year.
Inside the hotel, finance mandarins from 19 countries and the European Union, plus top officials of the World Bank and International Monetary Fund held talks. Formed in 1999, the G-20 includes the Group of Seven advanced industrial countries and the European Union as well as China, Brazil, India, Russia, South Korea, Argentina, Australia, Indonesia, Mexico, Saudi Arabia, South Africa and Turkey.
Together, the G-20 members represent about 90 percent of the world's gross national product, 80 percent of the worlds' trade and two-thirds of its population.
"The discussions here today about trade, about energy and about macroeconomic circumstances proves very clearly that this is a very important" forum, IMF Managing Director Rodrigo de Rato told reporters outside the meeting.
Activists criticized the governments and institutions represented by the G-20 on a wide front, including the Iraq war, the environment and Western capitalist systems.
One speaker, Margarita Windisch, called for the arrest of Paul Wolfowitz, the former US deputy defense secretary who is attending the Melbourne talks as World Bank chief, for his role in the US-led invasion of Iraq in 2003.
"He has got no right to walk the streets any more," said Windisch, waving a pair of handcuffs, to cheers from the crowd. "We will ask the police to lock up this war criminal."
During the unrest, a protester sprayed a branch of the ANZ bank with the words: "ANZ is a war profiteer" as another unsuccessfully tried to smash the building's window. One man briefly sprayed police with a fire hose.
Reform of the IMF, rising interest rates, the Chinese and Japanese currency levels and efforts to economically isolate nuclear-armed North Korea are also likely to come up at the closed-door meetings.
Surging demand for oil and minerals from fast growing economies China and India have benefited commodity powers like Australia, while fanning concerns over the emergence of unstable supplies and market distortions.
De Rato told reporters Saturday that governments have no power to affect the value of major currencies and should leave that task to markets.
"The markets are the ones who fix the value of the most important currencies and governments won't be able to affect that," de Rato said.
On Friday, US Deputy Treasury Secretary Robert Kimmitt renewed Washington's call for China to move faster in reforming its currency, the yuan.
"We believe that the Chinese need to accelerate the movement of their exchange rate to reflect underlying market conditions," Kimmitt told reporters.
A group representing major international mining companies, the Energy and Minerals Business Council, is due to address G-20 delegates Saturday - the first time a business group has had direct access to the forum.