Failure to tackle crime and preserve Britain's traditional civil liberties could foster "an evil extremism", Oliver Letwin has warned.

Speaking on Tuesday, the shadow home secretary vowed that a future Conservative government would support "the inhabitants of hard pressed estates" and police officers across the UK.

And he said the party would establish new sheriffs as part of a scheme to decentralise control over local police forces.

Vowing to "reclaim the streets", Letwin said the party would aim to put 40,000 more police officers on the streets than there were at the beginning of this year, to be recruited at a rate of an additional 5,000 a year.He said that cutting the cost of the asylum and immigration system could fund the policy.

"We will replace the present asylum system in its entirety - with a system of quotas for genuine refugees and the offshore processing of all claims, to deter all but genuine claims for protection from persecution," he said.

However, he accepted that if the planned European constitution comes into effect, the policy would have to be scrapped.

Stressing his belief in civil liberties, Letwin told delegates that key changes were needed to prevent the "triumph of evil".

"If there is one thing in the man made world I believe in, that thing is Britain's liberal democracy," he said.

"But we cannot and must not take the continuity of that precious liberty for granted."

He said that it was a failure to tackle "social and physical decay" that risked fostering by omission "an evil extremism that imperils our peace, our prosperity and our liberty".

Letwin said the government had tried to tackle rising crime, but had failed because Labour are "the only people in Britain who really believe in bureaucracy, who really think they can work it out from Whitehall".

He lambasted David Blunkett's Home Office for hiring 9,000 extra police officers but 10,000 additional civil servants.

And he said that a Conservative government would put police forces under the control of the public.

"We will give every chief constable a cast-iron legal guarantee of operational independence," said Letwin.

"And we will put each local police force under the direct, democratic control of local people."

The party plans to make chief constables answerable to directly elected sheriffs, mayors or police boards rather than the home secretary.

Details of the new structures would depend on approval in local referendums and the existing council structures.

Police forces would be given block grants, with the sheriffs of police boards deciding on local strategies and how to spend the money.