By Stefano Ambrogi and Tim Castle
LONDON (Reuters) - Breakaway protestors scuffled with police and smashed windows in central London on Saturday as tens of thousands of Britons marched against government austerity cuts.
Groups dressed in black and wearing masks threw flares and smoke grenades and broke into a branch of HSBC bank in the center of the capital.
Police were pelted with paint and what they said were light bulbs filled with ammonia by protestors who split from the main rally called by unions to protest public spending cuts, rising unemployment, tax rises and pension reforms.
They also cracked windows at a McDonald's restaurant and at another shop and bank branch.
Union leaders said well over 250,000 people angry at government economic policies joined what was billed as the biggest rally in the capital since protests against war in Iraq in 2003.
Around 4,500 police officers, some wearing riot helmets, were on duty along with hundreds of union-trained stewards.
The Conservative-led coalition is pushing ahead with a tough debt reduction programme to virtually eliminate a budget deficit, currently running at about 10 percent of GDP, by 2015 to protect Britain's triple-A credit rating.
Unions and the opposition Labor Party say the measures go too far, too fast and are bringing misery to millions of Britons with unemployment at its highest level since 1994.
Labour leader Ed Miliband told protesters in Hyde Park they followed in the footsteps of the suffragettes, the U.S. civil rights movement and the anti-apartheid movement.
"Our struggle is to fight to preserve, protect and defend the best of the services we cherish because they represent the best of the country we love," he said.
Brendan Barber, general secretary of the umbrella labor organization, the Trades Union Congress, said earlier he expected the march to be London's largest since up to 1 million protested against the planned invasion of Iraq in 2003.
Many European countries have seen mass protests in recent months as governments slash public spending to try to help their economies to recover from the global financial crisis.
The government says it is cleaning up a mess left by the previous Labor government and that failure to act would leave Britain exposed to market turmoil.
Retired journalist Karl Dallas, 80, said he had got up at 5:30 a.m. to travel to the march from Bradford in north England.
"The cuts are totally unnecessary," he told Reuters.
"Britain had a worse deficit after the (second world) war, we established the National Health Service, nationalized the railways and the coal mines, laid the basis of the welfare state. Now they are destroying the welfare state -- it's political, not economic."
(Writing by Olesya Dmitracova; editing by David Cowell)