Sir John Stevens says Londoners will not be cowed by terrorism

Scotland Yard - police headquarters in London - said Sir John had been travelling to the US as events unfolded on Tuesday but security restrictions meant his plane was forced to return to London's Heathrow Airport.

Thousands are feared to have died when hijacked passenger planes were forced to crash into the World Trade Center in New York and the Pentagon in Washington.

Another passenger jet - a United Airlines 757 - has crashed in Pennsylvania, south-east of Pittsburgh with all 45 people on board feared dead. It is not clear whether that too had been hijacked.

Security steps

Airports on alert All private planes grounded No flights over London Police across UK on alert Defence facilities across world on alert Business and finance institutions advised on extra measures Cooperating with US, EU and other allies on security issues Cabinet security meeting to be held on Weds morning

Sir John said he wanted to reassure the British public that police were taking "all necessary precautions" to ensure similar attacks did not happen in the UK.

"Should there be any warnings we will let people know but at the moment it is business as usual and everything will be done by the security forces of this capital to ensure that people are as safe as possibly can be in the circumstances."

He added: "We are working very closely with the security services and we are working extremely closely with our American colleagues and others."

Airports on alert

The UK's airports, cities and government buildings are on heightened security.

Heathrow Airport is a focus for particular attention, said Sir John.

All private aircraft have been grounded and no flights allowed on the normally busy flight paths over London until further notice.

Sir John said the air exclusion zone over London was a "precaution" to ensure only authorised aircraft could fly in the area.

Both Trade Centre towers collapsed

There is no intelligence that London or other parts of Europe are a target, he said.

Some officers have had their leave cancelled and others have been taken off training and put on operational duties to give the Metropolitan Police about 500 extra officers.

Sir John said Londoners were "not cowed by terrorism" and should try to carry on business as usual as far as possible on Wednesday.

He said it was important that the people responsible for the "horrendous" outrages in the US were identified as early as possible.