The incident, the second security breach at Heathrow within weeks, came a day before Queen Elizabeth II is to unveil a new terminal at the airport, amid protests by campaigners opposed to plans to expand it.

'A man ran towards an Emirates flight,' a police spokeswoman told AFP, after television pictures showed police vehicles scrambling to deal with the situation. 'He has been removed. We believe he was acting alone.'

A controlled explosion was carried out on a bag which the man dropped during the incident, police said. There was no sign of explosives, suggesting he may have been a protestor, they said.

BBC cited eyewitnesses saying the man scaled the airport's perimeter fence, ran onto the runway and was eventually surrounded by police.

One picture shown by Sky News appeared to show a policeman levelling a gun at the man as other colleagues dealt with him on the ground.

There was no indication that the incident was terrorism-related. Britain has been under an increased security alert since the July 7, 2005 suicide bombings which killed 52 people on the London transport system.

It is the second major security breach at the airport in the space of three weeks.

On Feb 25, four activists from environmental campaigners Greenpeace staged a protest on top of a parked jet against the planned construction of a third runway.

The latest incident came before the queen is due on Friday to open the new Terminal Five at Heathrow, designed to increase the airport's capacity to some 100 million passengers a year.

Buckingham Palace said the royal visit would go ahead as planned.

British Prime Minister Gordon Brown, told about the incident as he attended a European Union summit in Brussels, praised police handling of the situation.

'I think the important thing about the Heathrow incident is that the person was detained... I'm satisfied everything is now being done to ensure security at Heathrow is intact,' he said, in a statement to British television.

A number of domestic and short-haul flights were delayed. Among those affected were the Scottish women's football team. One person accompanying the team voiced frustration at the incident, saying: 'Surely there are better ways of protesting.'

German plastics engineer Jens Eisenmann, returning to his home in Heidelberg after a short business trip to Britain, was equally unhappy after his flight to Frankfurt was cancelled. 'It's a big problem for me,' he said.

Located to the west of London, Heathrow is one of the world's biggest airports, currently handling over 68 million passengers a year.

Protesters stepped up campaign against plans for new runway
Protesters have recently stepped up a campaign against plans for a new runway at the airport, which they say will damage the environment and add to misery for local residents.

Some flights were delayed because the northern runway, where the incident took place, could not be fully used.

A spokeswoman for the airport said: 'Airport police swiftly apprehended a man near the northern runway of Heathrow this afternoon.

'The northern runway is now being partially used while the police deal with the incident. The southern runway is fully operational. Some delays are occurring as a result.' -- AFP