Tens of thousands of Britons are expected to protest against the coalition government's austerity measures on Saturday in the biggest demonstration in the capital since the Iraq War in 2003.
Union leaders say more than 200,000 people could join the demonstration, angry at deep public spending cuts, rising unemployment, tax rises and pension reforms against a backdrop of falling or frozen wages and higher prices.
The Conservative-led government 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 in order to protect Britain's triple-A credit rating.
However, unions argue the measures go too far, too fast and are both economically unsound -- putting future growth at risk -- and are also causing misery to millions of ordinary Britons, with unemployment at its highest level since 1994.
"All the signs are (the march) is going to get huge support," said Brendan Barber, general secretary of the umbrella organisation, the Trades Union Congress (TUC).
He expected it to be the London's largest since up to a million protested against the planned invasion of Iraq in 2003.
"There's a kind of buzz around it. So 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 in an interview 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.
Prime Minister David Cameron insists he has no intention of reversing the government's policies and points to the backing of a number of British and global economic bodies.
The government says it is cleaning up a mess left by the previous Labour government, in power for 13 years until last May, and that failure to act would leave Britain exposed to market turmoil and ultimately prove more painful.
Police fear simmering resentment could boil over and 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.
Internet chatter indicates some splinter organisations 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.
Police chiefs say some 4,500 officers will be on duty along with hundreds of union-trained stewards.
Following criticism of police strategy at other recent mass protests, particlarly the use of the "kettling" containment tactic, officials from the human rights organisation Liberty will act as observers for the first time.
Len McCluskey, leader of Britain's biggest union Unite, said the protest would be the start of widespread action which would almost certainly result in coordinated strikes.