Two men have been arrested on suspicion of endangerment of an aircraft after RAF Typhoon jets were scrambled to escort a passenger plane over the UK.
Police boarded Pakistan International Airlines (PIA) flight PK709, originally en route to Manchester from Lahore, after it was diverted to Stansted.
The men being held are aged 30 and 41, Essex Police said.
Passengers said they had heard threats had been made on board, but there has not been any official confirmation.
However, PIA said all the passengers were safe.
A bomb disposal vehicle has arrived by the plane.
The plane was scheduled to leave Lahore - the capital of the Pakistani province of Punjab - at 09:35 local time (05:35 BST) and had been due to arrive in Manchester at 13:30 BST.
BBC defence correspondent Jonathan Beale said there was a suggestion there had been threats on board during the flight, involving some sort of weapon or bomb - although this has not been confirmed.
Emergency signal
However, one passenger told the BBC that the pilot had informed them there had been threats.
The man said: "We were about half-an-hour away from landing in Manchester and we saw that the plane was taking different actions.
"We did not know anything about it other than the pilots announced that they have landed at Stansted. And we landed, safely. Then he announced that he had a threat from someone, which was why he had landed the plane."
Another passenger, Umari Nauman, told Sky News that cabin crew had said two men had repeatedly tried to get into the cockpit.
"The cabin crew informed us that basically they tried to come into the cockpit a few times and because they had been asked not to do that, they got into a bit of an argument with the crew and made a few threats."
Stansted said the pilot had alerted the authorities to the incident, but the airport did not say what that incident was.
Speaking about the passengers, an airport spokesman said: "They will be taken to a reception centre in the airport where police may wish to interview them about what they have seen. At some point police and the airline will arrange for their onward transportation to Manchester."
Stansted earlier said on Twitter that it was operating normally.
A Manchester Airport spokesman said: "A Pakistan International Airlines aircraft which was due to land at Manchester Airport at 13.30 BST today has been diverted to our sister airport London Stansted, where the authorities are investigating."
The airport said it expected passengers to be put on a coach from Stansted to Manchester later on Friday.
Mashhood Tajwar, a spokesman for PIA, said the incident followed a call to air traffic control.
He said that about 20 to 30 minutes before landing, information received by air traffic control in Manchester had indicated there might have been "some security threat".
"But so far it has not been ascertained what was the real motive behind that call," said Mr Tajwar.
He added: "The plane was diverted to Stansted and it has landed safely over there and all passengers are safe, and they have disembarked."
PIA said there had been 308 passengers on board, as well as 14 crew including pilots, with a mixture of Pakistani and British passport holders.
'Serious incident'
The MoD said responsibility for the incident had passed to Essex Police and the Home Office, and that "our involvement is over".
An MoD spokesman said Typhoon jets could be scrambled after the pilot or crew of a passenger aircraft sends out an emergency signal.
"The purpose of going up is to investigate what the situation is," he said.
"Often when a quick reaction alert aircraft is launched the details are not known, but it is known that a signal has been sent."
Philip Baum, of Aviation Security International, told the BBC: "This is certainly a significant incident, however the fact that fighter jets were scrambled to intercept is not unusual.
"Fighter jets are scrambled when there is a bomb threat, when there is a hijack, when the wrong transponder code is used or if an aircraft cannot communicate with the ground. Often the result is when there is an unruly passenger on board.
"What is interesting in this case is that it would appear that the aircraft was only 10 minutes from landing - these are initial reports coming in - when they decided to divert to Stansted. So to actually change a destination 10 minutes from landing would certainly imply that there was some serious incident taking place."
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