From Australian paper The Age, reporter Guy Jackson explains the protests.
Police estimated the crowd at up to 35,000 but there was no sign of the feared violence as the placard-waving crowd snaked along the six-kilometre (four-mile) route to Hyde Park.
An alliance of more than 150 unions, charities and environment groups joined the march to demand action to save jobs, create a low-carbon economy and impose stricter controls on the finance sector.
World leaders, including US President Barack Obama on his first visit to Europe since he took office, will meet in London on Thursday for the Group of 20 summit amid the deepest global recession since the 1930s.
Organisers of the Put People First march for "jobs, justice and climate" had rejected as "smears" claims in police briefings that the march could be hijacked by anarchists bent on violence.
The general secretary of the Trades Union Congress (TUC), Brendan Barber, said the demonstration had a clear message for the presidents and prime ministers heading to London.
"Never before has such a wide coalition come together with such a clear message for world leaders," he said.
"The old ideas of unregulated free markets do not work, and have brought the world's economy to near-collapse, failed to fight poverty and have done far too little to move to a low-carbon economy."