By Michael Holden
LONDON (Reuters) - Tens of thousands of Britons opposed to government austerity cuts were expected to join the biggest rally in the capital since protests against war in Iraq in 2003.
Union leaders say more than 200,000 people angry at deep public spending cuts, rising unemployment, tax rises and pension reforms could join the demonstration on Saturday.
Police fear simmering resentment could lead to a repeat of violence seen last December when student protests against higher university tuition fees erupted into the capital's worst rioting in decades.
Some 4,500 police officers will be 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 Labour 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.
"People know we've got to get the deficit down... but to do things this far and this fast, to destabilize communities, pushing unemployment up, people are worried about that," Labour shadow chancellor Ed Balls, surrounded by protesters holding banners, told BBC television.
Brendan Barber, general secretary of the umbrella labor organization, the Trades Union Congress (TUC), 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.
"I think it's going to be a very, very big event and a kind of powerful message to the government they are losing public support," he told Reuters earlier this week.
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 Labour government and that failure to act would leave Britain exposed to market turmoil.
The Labour Party says it would reduce the deficit at a slower pace, aiming to halve it by the time of the next election in 2015.
"They have a secret plan of their own of 14 billion pounds' worth of cuts next month and they won't even level with people about where those cuts will fall," Deputy Prime Minister Nick Clegg told Reuters in an interview on Friday.
"Have they told their trade union paymasters, have they told the demonstrators, that Labour is now the only party that wants to go into the next general election advocating three more years of cuts?"
Internet chatter indicates some splinter organizations and anarchist groups are planning direct action, with occupations of iconic areas of central London, such as Trafalgar Square, the Oxford Street shopping district, and Piccadilly Circus.
(Additional reporting by Olesya Dmitracova; editing by David Cowell)