Police are attending a security alert at London's Heathrow Airport, a spokesman said.
One passenger told Sky News armed police boarded a flight, believed to have been bound for Dubai, and took a man off in handcuffs.
A Metropolitan Police spokesman said they were aware of the incident but the airport remained open.
Cameron McLean said: "Police just swarmed the guy and rushed him out.
"I think he was a white male. There was another one but I didn't see him."
He said the man was taken out in handcuffs then sniffer dogs were brought on board.
A Scotland Yard source confirmed the incident occurred on a plane, believed to be an Emirates jet.
A spokesman for the Met said: "We are aware of a security incident at Heathrow. The airport and terminal remains open."
An airport spokesman said: "I can confirm there has been an incident on the EK004 flight to Dubai.
"Police are attending. It's not affecting the rest of the airport which remains opens."
