Map of Parliament's security measures

Whitehall officials and the Metropolitan Police insist the action is not in response to a specific threat, but part of a continuing process to increase security in London.

There will also be more visible policing at Heathrow airport.

In the past few weeks, terrorist attacks in Saudi Arabia and Morocco have raised fears that al-Qaeda has regrouped and is again planning to strike western targets.

The BBC's security correspondent Frank Gardner said the primary aim of the increased security "is to stop a suicide truck bomb slamming into the Houses of Parliament", which he said would have been incredibly easy.

The concrete blocks would remain in place for a long time, he added.

The security of Westminster remains under constant review in the light of the changing assessment of the terrorist threat Lord Brabazon

"It is not going to stop a determined attacker with really subtle carefully worked out plans, but it is certainly going to deter the casual bomber," he said.

Parliament, however, remains open for visitors during the current recess.

In the last 24 hours Eliza Manningham-Buller the head of MI5, Britain's security forces, has met with George Tenet, the director of the US Central Intelligence Agency (CIA).

They were believed to be discussing the increased threat of terrorism from the al-Qaeda network.

Extra security is also to be set up at Heathrow Airport. Tanks were deployed at the airport for a few days in February.

'Alert not alarm'

Security at Parliament has been tight since the 11 September attacks in the US, with armed police on guard outside the main entrances.

Lord Brabazon of Tara, chairman of the Lords administration, announced the security arrangements on Thursday.

What are they going to achieve? Patrick Hudson visitor from Portsmouth Views on Parliament's increased security

"The security of the palace (of Westminster) remains under constant review in the light of the changing assessment of the terrorist threat," the crossbench peer said.

He added that more security guards would be positioned on the West Front at Westminster as well as the concrete blocks.

A spokesman for Scotland Yard said the message to the public was "alert not alarm".

"These measures are being carried out on a precautionary basis in light of events around the world and the fact that security in the capital remains at a high level.

Policing has increased at Heathrow

"There is no specific intelligence of places, events or people in the UK that would lead us at the moment to issue specific warnings to the public.

"We want to reassure Londoners that their safety is our primary concern."

Liberal Democrat home affairs spokesman Simon Hughes urged people not to panic about the new measures, but said the government should be more open about such moves.

Information plea

"Response to terrorism requires sensible precautions but nothing which encourages panic," he said.

"The appearance of these latest protective blocks should remind the authorities that the more public information they can give the more the public can understand.

"Better protection and greater vigilance may be necessary. But more information for the public is as useful a defence as anything."

Their target is the whole fabric of Western society - absolutely everything is a target Security expert Charles Shoebridge

Former anti-terrorism officer and security expert Charles Shoebridge believes a terror attack on Britain is now inevitable.

The war in Iraq had been a "recruiting drive" for al-Qaeda and it now had an "unlimited supply" of suicide bombers, he told BBC News.

Al-Qaeda's main weapon was "the use of large vehicle-borne bombs", Mr Shoebridge said.

Parliament's "symbolic value" made it a major target, but he added: "Al-Qaeda's target is the whole fabric of Western society - absolutely everything is a target."