Essex Police say they boarded a passenger plane diverted to Stansted Airport and two men have been arrested on suspicion of endangerment of an aircraft. They have been removed from the plane.
Police and partners are continuing to respond
Stansted says passenger flights in and out of the Essex airport are operating normally.
It follows the arrival of a Pakistani International Airlines flight which was diverted from Manchester and escorted in by military jets.
A British security source has told Reuters news agency that early indications do not suggest that the plane is the subject of a terror attack.
It is understood the incident on board the Pakistan International Airlines flight PK709 happened around 10 minutes before the plane was due to land in Manchester at 2pm.
Stansted Airport has said that the diverted plane is in an isolated stand away from passenger areas.
It added that the airport is operating as normal.
Eurocontrol, Europe's air navigation safety watchdog, said "significant" delays are expected at Stansted Airport following an "incident with a 777 declaring an emergency":
Two Typhoon jets from RAF Coningsby have been scrambled to escort a passenger airliner.
The Pakistan International Airways jet has been diverted to Stansted airport which is the UK's designated counter terrorism airport.
Defence sources cannot yet say why the jets were scrambled to assist. The RAF carries out around a dozen such missions each year.
The passenger aircraft has now landed at Stansted.
The aircraft, which is still in the air, is a Pakistan International Airlines passenger plane which was flying from Pakistan to Manchester, a Manchester Airport spokesman said.
The passenger flight (PIA709) that has been diverted is a Pakistan International Airways flight from Lahore in Pakistan to Manchester.
The Boeing 777-340ER, which is reportedly being diverted to Stansted Airport, has a capacity of 300-550 passengers.
The Pakistan International Airlines plane that has been diverted from Manchester Airport to Stansted is carrying 297 passengers, Reuters reported.