LONDON, Sept 7: Britain has been put on red alert to prevent possible terrorist attacks on the anniversary of the Sept 11 attacks in United States.

The government has taken unprecedented security steps.

On Wednesday, armed roadblocks will be in place around London. A number of Muslim extremists are being kept under surveillance, it has been learnt. The security services and police will keep a close check on people regarded as a potential danger, who have not already been interned or arrested.

Officials said there was no specific threat to the UK but warned that lone terrorists unconnected to Al Qaeda could strike in a bid to exploit the anniversary of the terror attacks in New York and Washington.