Around 250,000 people are believed to have taken part in the TUC's anti-cuts rally across central London.
The enormous scale of the 'March for the Alternative' became clear throughout the early afternoon as a stream of demonstrators worked its way from Whitehall and parliament up to Piccadilly.
Having set off from Victoria Embankment at 11:30 GMT, marchers were continuing to pass by parliament four hours later.
TUC leader Brendan Barber told those assembled in Hyde Park that "the coalition government has turned out to be a demolition government".
"Let's be brutally clear about these brutal cuts. They're going to cost jobs on a huge scale - adding to the misery of the 2.5 million people already on the dole," he said.
"They're going to hammer crucial services that bind our communities together. And they're going to hit the poorest and the most vulnerable hardest. Anyone who tells you different is a bare-faced liar.
"The government claims there is no alternative. But there is. Let's keep people in work and get our economy growing. Let's get tax revenues flowing and tackle the tax cheats. And let's have a Robin Hood Tax on the banks, so they pay us back for the mess they caused."
Labour leader Ed Miliband compared the gathering to the marches against apartheid as he gave a speech to those present.
"The Tories said I shouldn't come and speak to you today, But friends, I am proud to stand with you, because we know there is an alternative," he said.
Trade union organisers had expected upwards of 100,000 people to arrive in London. Some reports suggested as many as 500,000 people could be marching today, but the Met claimed 250,000 was a more accurate figure.
Some 4,500 officers were on the streets of London to police the demonstration, in one of the biggest protest operations the Met has organised in years.