Activists had initially gathered along Victoria Embankment to stage a protest for "jobs, justice and climate". Just after noon, they began their four-mile march to Hyde Park, with most marchers arriving at about 2pm.
Some protesters chanted: "What do we want? Jobs not bombs". Many carried banners bearing slogans including "People before profit”, "Money for need not greed” and "Climate emergency".
Demonstrators whistled and booed as they made their way past 10 Downing Street along Whitehall.
The event, which took place on a damp, cold and blustery day, has been organised by an alliance of more than 150 unions, environment, charity, faith and development groups.
The Put People First march, which comes amid anger at the £19 million cost of staging the conference at a time of economic downturn. Some marchers wore hoods, face scarves and dark sunglasses in an apparent attempt to hide their identities.
The Metropolitan Police estimated that between 12,000 and 15,000 people were on the march. By early afternoon, there had been no arrests and the mood among demonstrators was largely upbeat.
The demonstrators were confident, however, that they had sent a strong message to the world leaders who arrive for the summit on Wednesday.
Those protesting included climate change activists, anti-war campaigners, trade unionists, charity workers and students will join forces in the march.
There was one rowdy group of 100 anarchists who chanted: "Harry Roberts, he’s our friend. He kills coppers." Roberts was jailed for a minimum of 30 years at the Old Bailey in 1966 for murdering three unarmed officers.
As the protesters passed the Ritz hotel in Piccadilly, they chanted: "The Ritz, the Ritz. We are going to get rid of the Ritz." The hotel’s ground-floor windows were boarded up as a precaution against being targeted.
Dominic Eagleton, 37, who works for Action Aid, was one of the marchers. "People like me are here today to tell the G20 leaders that tinkering around the edges won’t work. There has to be a systematic transformation of the system towards something more socially and economically just that’s not only in the interest of big business but in the interest of ordinary people," he said.
"It’s not just a small bunch of radicals calling for this today. This is a feeling shared by many ordinary people."
The march also attracted many individuals of no party or group moved to join the protest by their anger at the injustices of the world economy.
Margaret Martin, 62, a retired nurse from Witham, North Essex, said: "I want the world’s leaders to consider the poor so that people in the third world will benefit from what’s agreed at the summit."
Vernon Cowdy, 66, a retired actor and civil servant from south east London, said: "Some controls have to be devised on the way capitalism works. The G20 leaders need to hang on to the idea of rebalancing things to create a system that’s fairer to all."
Leanne Bird, 24, a member of Friends of the Earth said "I’m here to show concerns about climate change and its destructive effect on our planet and on people’s lives. We can’t keep going on like this we have to use green energy and build a whole new economy."
The demonstrators carried placards reflecting their wide range of concerns. "Workers for the world unite", said one. "Brown out, green power", said another.
Others called for a free Palestine and for Western troops to pull out of Iraq and Afghanistan.
Harry Blissett, 46, a factory worker form Lincolnshire, said "this is a protest against the government they have betrayed the working class and are just looking after the middle class. We want justice and an end to poverty."
Bob Crow, the leader of the RMT Rail Union, said the protest would put Hugh pressure on the G20 leaders to create a different kind of society. "They are giving trillions of pounds to the bankers while ordinary people are being thrown on to the doll.
"We need more housing, better pensions and more jobs or we will end up with a mass unemployment we had in the 1930s."
Peter Rowland, 57, a rail guard from Manchester, said "The government has to do something for everyone not just the bankers. It is our money there giving to the bankers."
Ceri Richards, 29, a charity worker with Save the Children, said "We are here to call on the G20 to put children on their agenda. It is their responsibility to help the most vulnerable people in our society. Children must not be forgotten."
Ed Miliband, the Climate Change Secretary, met campaigners before the march got under way this morning and said the "vast majority" were planning a peaceful protest.
The Rt Rev Richard Chatres, the Bishop of London, said the atmosphere at the event was "positive" this morning, adding that it would be a "tragedy" if events turned violent.
The protest will target bankers and financiers they blame for the worst global recession of the past 50 years.
Thousands of police officers were on duty in central London in the days preceding the summit on Thursday and to deal with any violence in the run-up to the conference.
The Metropolitan Police fears some elements within the protesters’ ranks that are intent on violence.
Senior commanders are particularly concerned about the main day of protest on Wednesday, April 1, and have insisted that they are "up for it, and up to it", should there be any sign of disturbance. The force has refused to rule out the use of anti-terror legislation.
But organisers of the protest rejected as "smears" claims that the event will be anything other than peaceful and law-abiding.
They claim police briefings have given the impression that the demonstrations will be mounted by anarchists intent on causing trouble.
David Howarth MP, who is leading a Parliamentary group of observers at the protests, said: "I am increasingly worried that what the police are saying about the protests will end up in a self-fulfilling prophecy.
"By talking up the prospect of violence they will put off peaceful protesters and start to attract other sorts."
Andrew Dismore MP, who chairs the joint common human rights, said police language in recent days had been "not very helpful".
He said: "The police have a duty under the Human Rights Act to facilitate protest and not frustrate it.
"If they act in a confrontational way and use confrontation language, they will start to provoke the kind of behaviour they are seeking to prevent. There may well be a fringe element that want to incite violence.
"But that doesn’t mean police should criminalise every protester."
Saturday’s march will culminate in a rally at Hyde Park, which will hear calls for action to save jobs, support a low-carbon economy and stricter control of the finance sector.
Brendan Barber, the TUC general secretary, said: "Never before has such a wide coalition come together with such a clear message for world leaders.
"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.
"Of course the G20 will not solve everything in a day’s work, but leaders must sign up to both boost the world economy and govern it better, and show us that they are trying to build a better world."
Some of the world’s most famous public buildings and landmarks will switch their lights off at 8.30pm on Saturday as part of a global gesture against climate change.
These include Buckingham Palace, Big Ben, the Houses of Parliament, the Eiffel Tower, Sydney Opera House and the Millennium Stadium in Cardiff.
But some militant protesters have threatened to force the light out at business headquarters which do not voluntarily take part in Earth Hour, with plans for action against "wasted energy" in London office blocks over the weekend. The Telegraph (UK)