Police said they dealt with a number of incidents of "anti-social behaviour" as breakaway groups of anarchist protesters protested outside major companies including McDonald's and Starbucks in the Oxford Street shopping hub.
Scotland Yard did not provide an estimate for the turnout on the 4.8-kilometre march route, but organisers said police had told them around 100,000 people attended.
"This is not a crisis that is going to sort itself out through cuts," 19-year-old protester Jonathan said.
"We've had a double-dip recession now, and we are here today to show we are not going to stand it any longer."
In Scotland's biggest city Glasgow around 5,000 people took part in a separate protest while there was also a march in Belfast, Northern Ireland.
Britain climbed out of a deep economic downturn in late 2009 but fell back into recession at the end of 2011.
The coalition said after coming to power in 2010 that most ministries' budgets would be be cut by a fifth over four years, while other unpopular measures include a tripling of university tuition fees and a public sector pay freeze.
Protesters paused to boo at Mr Cameron's Downing Street residence, and shouted "Pay your taxes!" at a Starbucks coffee shop.
Starbucks was embroiled in a row this week after it was reported that the US giant paid just $13.8 million in British corporation tax over 14 years.
A smaller group of protesters, many of them masked and clad in black, gathered in Oxford Street to demonstrate against big brands accused of tax avoidance.
"There's plenty of tax-avoiding businesses round here and they're being targeted today - companies like Starbucks, Topshop and Vodafone," 18-year-old protester Johnny, a student at London's Goldsmiths University, said.
Dozens of police in high-visibility jackets stood in lines outside branches of McDonald's, Starbucks and other firms, blocking protesters from entering, while police helicopters buzzed overhead.
At a huge rally in Hyde Park at the end of the march, opposition Labour leader Ed Miliband attacked Mr Cameron for "cutting too far and too fast".
"He clings to an economic plan that isn't working," Mr Miliband told protesters.
But the crowd booed Mr Miliband when he said that any government in power at the moment would have to make some spending cuts.
Mr Cameron, whose Conservatives share power with the centrist Liberal Democrats, insisted that spending cuts were needed to balance Britain's budget.
"Today Ed Miliband is headlining a rally calling for an end to every single spending cut needed to clear the deficit," he said on his Twitter account.
The protest comes after the Conservatives were hit on Friday by a fresh barrage of accusations they are a party of the rich and are out of touch with ordinary voters.
abc.net.au 21 Oct 2012
Protests like this will soon become illegal.
London is officially the city with the most surveillance cameras.
All the protesters have been tagged, and this information has been stored for future reference.