A replacement Singapore Airlines (SIA) aircraft is on its way from Singapore to Baku, Azerbaijan, to pick up passengers and cabin crew who are stranded at the airport there.
They were all on board an SQ plane that had to make an emergency landing there due to depressurisation.
The aircraft lost pressure about six hours into the flight.
A passenger related on what actually happened during the incident.
Nathan Phelps, a passenger, said: "I think we were just going over the mountainous area of Pakistan... and then all of a sudden, the plane just literally dropped real, real fast. As I had it, I had the map on the screen and you could see the altitude dropping rapidly. I think we dropped about 20,000 feet in less than two minutes.
"Then the mask dropped and then "Emergency, Emergency" came on the tannoy and then we were told to put our masks on. There was a bit of panic, I wouldn't say they were overly panicked. To be honest if anything, it was quite quiet, it was eerily quiet, the plane was quite dark. The captain just basically said there was a pressurisation problem on the plane."
Flight SQ317, an Airbus A380 aircraft, was travelling from London to Singapore before it had to be diverted to Baku.
There were 467 passengers and 27 crew members on board the plane. No one was injured.
SIA said the aircraft landed "uneventfully" at Baku Airport at 1.03am local time.
The replacement aircraft is expected to reach Baku at 11pm local time there and arrive back in Singapore on Tuesday afternoon.
SIA said it doesn't operate regular flights to Baku.
Staff from both Istanbul and Moscow have been deployed to provide assistance on the ground.
An aviation expert explained the likely causes of depressurisation.
Assoc Prof Randy Chue, School of Mechanical and Aerospace Engineering at Nanyang Technological University, said: "Most common ones are such as, you have leak in the doors, there may be cracked windows.
"The more serious cases would be the systems maintaining the cabin pressure (that) are not working properly, but usually those are very rare. It is usually the seals of the door that are the culprits. Although it may be panicky for the passengers, usually it does not cause any injuries."
Earlier, SIA said it is seeking clearance from local authorities to transfer affected customers to a hotel while waiting for a replacement aircraft to arrive.
But passengers said they were not allowed to leave the airport as they did not have a visa.
And they said it took about two-and-a-half hours before food was brought out from the plane.
Singapore Airlines has apologised to affected customers for the inconvenience and said it is investigating the cause of the depressurisation.