Tens of thousands marched for five hours in "one of the largest" protests in London on Saturday, four days before the G20 summit kicks off.

Leaders from the world's 20 biggest economies are scheduled to meet in London on April 2. They will discuss how tighter regulation of financial markets, billions of dollars in stimulus measures and credit lines for international trade can help the world economy recover.

The police, who are coordinating a large-scale security operation in the build-up to the summit, have been barred from going on leave until the summit ends.

"It's fair to say that this [protest] is one of the largest, one of the most challenging and one of the most complicated operations we have delivered," Commander Simon O'Brien, one of the officers in charge of security, said of Saturday's protest.

While the protest from Temple Place to Hyde Park highlighted several issues, including social inequality, loss of jobs, war and climate change, most took to the street to express their anger over bankers, whose greed they say led to the global financial crisis.

The protesters, ranging from union members, taxi drivers and the retired, marched the 7.2-km stretch to the sound of brass bands, piercing whistles and blasting music from stereos. Among them was 73-year-old Geoff Stratford who awoke at 4:30 am so he could arrive in London from Manchester in time for the rally.

"The cause [of the financial crisis] lies in casino capitalism and in the greed of bankers," Stratford told China Daily as he and his wife marched, holding a banner.

"We want to get the view through to the leaders who are meeting to find consensus. We have pensions and are not affected much by the crisis but the leaders should correct their mistakes and make people suffer less," he said, adding that he would join the marches scheduled for Wednesday and Thursday as well.

Essex resident Milton McKenzie said: "How can we have a situation where we have people out of work and the bankers just cream it off, helped by the government?"

Dressed in green sportswear, Andrea Porteous and her 10-year-old daughter Taylera raised chants of: "Put children first".

Unwilling to admit she had lost her job, the 40-something former office administrator said: "I finished my work days ago." Porteous conceded that she had sent her resume to several companies but failed to get a single job offer. "Some of my friends too have not got feedback after sending 20-30 job application letters."

A London taxi driver said he joined the protest because the financial downturn meant his daily fare had gone down by 20-30 percent. "I have to drive six days a week or a minimum of two extra hours every day," said John, who refused to give his full name. "I am urging the G20 leaders to find a way to punish those who brought about the crisis."