About 35,000 demonstrators marched through central London on Saturday ahead of next week's summit for the Group of 20 nations in Britain's capital.
Protesters demanded action on issues ranging from climate change and the war in Afghanistan to the financial crisis. Those leading the march carried a large banner that read, "Jobs, Justice, Climate."
The demonstration ended with a rally in Hyde Park organized by labour unions, faith groups and charities under the title "Put People First."
Elsewhere, thousands of demonstrators marched through other European cities, including Paris, Geneva, Rome and Vienna, to demand that governments focus on jobs, the economy and the environment.
Some 15,000 people gathered in Berlin, and a demonstration also was held in Frankfurt, Germany's banking capital, under the slogan: "We won't pay for your crisis."
In Paris, the main banner read, "People before bankers."
Protest organizers said they wanted leaders from the world's top 20 economies to adopt a more transparent and democratic economic recovery plan as they gather for their one-day summit on Thursday.
Brendan Barber, who heads an umbrella group for Britain's unions, told assembled protesters in Hyde Park that the G20 needed to "take actions to lay the foundation for a better world."
"If we can generate fabulous wealth, as we can, then surely we can learn how to distribute that wealth more fairly. If we can unleash a technological revolution, then surely we can ensure that everyone on this planet gets the food, the shelter and the health care that they need," he said.
G20 leaders will be meeting in London's Docklands. Police say there will be road closures and other disruptions to transportation as a result of both the protesters and the visiting dignitaries.
The city's financial district will be the scene of large demonstrations starting Wednesday, with marches on Britain's central bank, the U.S. embassy and other prominent landmarks.