LONDON - Thousands of people marched through London yesterday to demand punishment for bankers, power to the poor, and protection of the environment at a mass protest meant as a wake-up call to world leaders gathering this week for an economic summit.

It was one of the largest demonstrations this city has seen since massive rallies against the invasion of Iraq six years ago. The turnout, estimated at 35,000, reflected the depth of popular anger over Britain's economic crash and the perceived greed of bankers and other high-fliers whom many people blame for the crisis.

Beneath a sea of banners, marchers from trade unions, charities, environmental groups, and churches snaked through the streets to converge on London's Hyde Park. Placards called for "people before profits" and "jobs, not bombs," in a nation suffering its worst unemployment in more than a decade.

"It's people that make changes, not governments," said Gail Cartmail, assistant general secretary of Unite, one of Britain's biggest unions. "There should be more investment in public services and housing, not less, [and] greater help to get people back into work."

The leaders of the Group of Twenty Finance Ministers and Central Bank Governors (G-20), an organization representing the world's biggest developed and emerging economies, will meet in London on Thursday to discuss ways to pull the global economy out of its hole. The meeting will be the first major summit for President Obama, who is hugely popular in Europe, but is having trouble reaching an agreement with European leaders on how best to combat the global recession.

In Berlin, raucous protests drew thousands of participants, and hundreds congregated in Paris. The London demonstration attracted protesters from across Europe, including 11.11.11, an umbrella organization of Belgian volunteer groups.

"We want governments to give a voice to poorer countries," said Benedikt Raets, who runs the organization's Web site. "The poorest countries are paying the highest price."

Yesterday's protest in London offered a foretaste of further demonstrations planned for this week - and of the security nightmare they present for London police. Several activist organizations have scheduled events around the city for Wednesday, when many of the G-20 leaders are set to arrive, and some underground anarchist groups have threatened to storm buildings in the City, London's financial district.

All leave has been canceled for police officers in order to mount a full-scale security operation that could cost more than $10 million. Antiterrorist and intelligence agencies are also on high alert.

But critics accuse the police of overstating the threat of violence to justify the use of heavy-handed tactics. Last week, influential members of Parliament expressed concern, saying that the police have bullied and harassed peaceful demonstrators in the past, including filming them and invoking antiterrorism laws against them.

Many activists agree that the crisis offers perhaps the best chance in years to get across their messages of greater social justice and environmental vigilance, which people all too easily brushed aside in their rush to buy stocks and SUVs during the boom times.