A major security alert has taken place at Heathrow airport as a man with a rucksack scaled the perimeter fence and ran into the path of an aircraft.
Eyewitnesses told the BBC armed police surrounded the man on the north runway.
Police said a man was arrested and a controlled explosion of a bag found nothing suspicious. They later said the incident was not terrorism-related.
The alert, which delayed some flights, came a day before the Queen opens Heathrow's new Terminal 5 building.
BBC correspondent Ben Ando said police had received numerous 999 calls at 1405 GMT from members of the public.
Reports were of a man of Asian or Arab appearance removing his shoes before scaling the airport's perimeter fence, he said.
It is understood a message sent from the pilot to air traffic control alerting them to a man on the runway may have been overheard by the police, allowing them to act swiftly.
Up to 20 emergency services vehicles were reportedly sent to the scene where the rucksack was recovered and the controlled explosion was carried out.
A Metropolitan Police spokeswoman said a man was being held in custody.
Buckingham Palace confirmed the Queen's visit to Heathrow to open the new £4.3bn T5 building would go ahead on Friday as planned.
Prime Minister Gordon Brown said: "I'm satisfied everything is now being done to ensure security at Heathrow is intact.
"We are determined to protect all passengers and all staff who go through Heathrow and every other airport."
Norman Shanks, a former head of security at airports operator BAA, said it was not the first time a person had climbed over airport fencing.
But he said that while it was possible to climb such barriers, airports also had patrols, surveillance and fast-response units in place.
Last month protesters opposing airport expansion plans breached security to unveil a banner on top of a parked passenger jet.
Campaign groups Greenpeace and Plane Stupid both denied any involvement in this latest incident.
Aviation security expert Chris Yates described it as "extraordinarily serious" and said a lot of hard questions would now be asked of Heathrow's security.
The northern runway, also identified as 27R, was temporarily closed but is now fully operational. The southern runway remained open throughout the incident.
A statement from BAA said the incident ended just before 1600 GMT.
"Some short delays have occurred as a result, but we expect to return to normal later this afternoon.
"Clearly this incident is a matter of some concern, and will be fully investigated in conjunction with the police," the statement said.
Passengers were being advised to contact airlines to find out whether flights have been delayed or cancelled.