Unions predicted some 400,000 public sector workers would walk out, a smaller protest than in November when Britain saw the biggest strike in years, but a significant show of public discontent just after Prime Minister David Cameron's government took a drubbing at local elections.

The government said only about 100,000 had taken part and dismissed the action as "futile".

However, the sight of some 20,000 police officers in black caps marching through London will be particularly embarrassing for Cameron, whose centre-right Conservatives pride themselves on being the party of law and order.

Deep cuts to police budgets and a government-commissioned report that recommended allowing officers to be sacked, pay reductions for some and raising the pension age, have all caused disquiet.

"I feel like the government has misled the public. It's nothing to do with making a leaner, more efficient police service," said Anthony Coultate, 32, a sergeant from Leeds in northern England.

The officers, whose caps bore the slogan "Cutting the police force by 20 pct is criminal", marched slowly past the interior ministry and other government buildings, blowing whistles.

Gareth Rees, 35, who suffered serious injuries while on duty requiring nine operations and three years of treatment, said he would have lost his job under the proposed changes.

"British policing could change forever if these changes are allowed to be pushed through and ultimately it's going to be the public that will lose out," he told Reuters.

DEEP SPENDING CUTS

The Conservatives and their junior Liberal Democrat partners have vowed to press ahead with unpopular austerity plans despite both parties suffering badly in local council elections last week amid discontent that the country had fallen back into recession after two years of deep spending cuts.

Public hostility to austerity has hit governments across the European Union, with French President Nicolas Sarkozy ousted by Socialist Francois Hollande who promised a gentler approach on debt reduction , and Greece in political crisis after voters deserted the main parties.

Nick Herbert, the minister in charge of policing, defended the government's planned reforms.

"It's very important that tough decisions are taken to deal with the deficit and the police service, police officers, I'm afraid, can't be exempted from that. I really don't think that would be fair," Herbert told Sky News.

Police officers have been legally barred from taking industrial action since the 1990s.

The Police Federation, which represents 135,000 low-ranking officers in England and Wales, said Thursday's action was set to be larger than the 2008 protest over a pay row with the then Labour government.

"We're at the lowest ebb I can ever remember," Paul McKeever, the federation's chairman, told Reuters.

"We're not against change," McKeever said. "What we're against is ill-informed change based on ideology which is going to damage the service, damage officers and most importantly damage the public as well."

On Wednesday, the government announced it was pressing ahead with proposals to overhaul public sector pensions. Those plans prompted one of the most widespread strikes ever seen in Britain last year.

Thursday's protest is unlikely to be the end of the action. Len McCluskey, general secretary of Britain's largest union Unite, has already warned that public spending cuts justified action during London's Olympic Games which start in July.

(Additional reporting by Li-mei Hoang; Editing by Louise Ireland)

• A Permanent Economic Emergency