A protester carrying two knapsacks was arrested Thursday after hopping a perimeter fence and running into the path of an airplane at London's Heathrow Airport, authorities said.

There was a controlled explosion on the bags and early reports are that no explosives were found, Sky News reported.

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.

Police said the man was spotted on the northern runway just after 2 p.m. (1400 GMT).

The suspect was arrested by armed officers and a police spokeswoman said it was too early to say what the bag contained. Several flight delays were reported.

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.

On Feb. 25, five environmental activists were arrested after they climbed onto a plane at Heathrow to protest plans to build a third runway there.

The Greenpeace protesters wrapped a banner around the tail fin reading: "Climate Emergency — No Third Runway."

Queen Elizabeth II is due to visit Heathrow on Friday to open the airport's new Terminal 5.

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The Associated Press contributed to this report.