By Stefano Ambrogi and Tim Castle
LONDON (Reuters) - Black-clad, masked youths clashed with police, smashed windows and started a fire in central London on Saturday when more than a quarter of a million Britons marched in protest against government spending cuts.
Breakaway groups splintered from the main rally and threw flares and smoke grenades and broke into a branch of HSBC bank in the center of the capital.
Hooded figures climbed on to the roof of luxury food store Fortnum & Mason while other protesters started a fire in the center of Oxford Street, the capital's main shopping street.
The clashes, although sporadic, rippled across the center of the city, and overshadowed a rally called by unions to protest against unemployment and public spending cuts, tax rises and pension reforms introduced by the Conservative-led coalition.
Union leaders and police said over 250,000 people joined the biggest rally in the capital since protests against war in Iraq in 2003.
The coalition, in power since last May, is pushing ahead with a tough debt reduction programme to virtually eliminate a budget deficit, running at about 10 percent of GDP, by 2015.
Prime Minister David Cameron's government says it is cleaning up a mess left by the previous Labour government and that failure to act would expose Britain to market turmoil.
Treasury minister Justine Greening condemned the violence.
"It's a real shame and totally unacceptable that this minority of people are committing criminal acts," she told Sky News.
Police were pelted with paint and what they said were light bulbs filled with ammonia by protesters in clashes which mirrored violence late last year over higher student tuition fees. Police said they had arrested nine people.
"Unfortunately we have had a group of approximately 500 criminals committing some disorder, including throwing paint at Topshop in Oxford Street and at the police, and scaring the public who are trying to shop," London Police Commander Bob Broadhurst said.
"That has been concerning but we are on top of it."
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.
Unions and the opposition Labour Party say the government measures are bringing misery to millions of Britons with unemployment at its highest level since 1994.
Labour leader Ed Miliband told marchers in Hyde Park that the government was taking Britain back to what he said were the divisive politics of the 1980s when Conservative Margaret Thatcher was prime minister.
"There is a need for difficult choices, and some cuts," Miliband said. "But, this government is going too far and too fast and destroying the fabric of our communities."
(Writing by Olesya Dmitracova and Keith Weir; editing by David Cowell)