Tens of thousands of people marched across central London Saturday to demand jobs, economic justice and environmental accountability, kicking off six days of protest and action planned in the run-up to the G20 summit next week.

More than 150 groups threw their backing behind the "Put People First" march. Police said around 35,000 attended the demonstration, but there were large gaps in the line of protesters snaking its way across the city toward Speaker's Corner in Hyde Park.

The marchers are pushing for a more transparent and democratic economic recovery plan.

"The whole economic meltdown ... There's a really good opportunity for governments to get together and invest in a sustainable future," said unemployed Steve Burson, 49, marching with the protesters.

The biggest groups backing the demonstration include the Stop The War Coalition, whose supporters marched under the slogan "Jobs Not Bombs," Friends of the Earth, and the Trades Union Congress, an umbrella group of British trade unions, which is calling for Britain's crisis-hit manufacturing base to share in country's banking bailout.

"They should be solving (the crisis) in the interest of working people," said Andy Bain, the president of Transport Salaried Staffs' Association. "All the money is going to the rich."

Protesters whistled and booed British Prime Minister Gordon Brown's 10 Downing Street office — with one shouting: "Enjoy the overtime!" as they filed past.

Security was tight around a small group of people waving anarchist flags Saturday. Anarchists and others have promised violence before the G20 meeting Thursday, and the British capital is bracing for a massive police operation as representatives of the world's 20 leading economies fly in for a summit on the financial crisis. More protests are planned Wednesday and Thursday, while left-leaning teach-ins, lectures, and other demonstrations are scheduled throughout the week.

Other demonstrations aimed at the G20 summit took place in Europe on Saturday.

Berlin police estimated that around 10,000 people gathered in front of the capital's city hall and more than 1,000 in Frankfurt, Germany's banking capital, for similar demonstrations under the slogan: "We won't pay for your crisis."

Some demonstrators in Berlin sported headbands reading "pay for it yourselves" and carried placards demanding: "make capitalism history."