British police arrested a man carrying a knapsack who scaled a fence at London's Heathrow Airport and ran onto a runway toward planes Thursday, authorities and witnesses said.
Police determined there were no explosives in the bag after a controlled blast was carried out on it.
One of the airport's two runways was partially closed after the security alert, and a Heathrow spokeswoman said some flights were being delayed. No airport buildings were evacuated.
The man was spotted on the northern runway just after 2 p.m. local time, police said. Witnesses said he scaled a perimeter fence before running toward aircraft.
Queen Elizabeth II is due to visit Heathrow to open the airport's new Terminal 5 on Friday.
Plane Stupid, a protest group demonstrating against airport expansion in Britain, said it was not involved and was not aware of any planned protest at the airport. Five members of the group breached security at the Houses of Parliament last month, scaling the building's roof.
Environmental group Greenpeace also denied involvement. On Feb. 25, five Greenpeace activists were arrested after they climbed onto a plane at Heathrow to protest plans to build a third runway there.
Barbara Reid, a council member in the London borough of Hounslow near Heathrow, said the security breaches at the airport were becoming worrisome.
"These things call their security procedures into serious question," she said.
