Large-scale disturbances are taking place in the UK's largest immigration detention centre.
Police, prison officers and fire crews were called to the Harmondsworth centre, west London, in the early hours after a number of fires were started.
About 50 detainees have been seen in a courtyard spelling out the words help and SOS with bed sheets.
A prisons' watchdog report criticised the centre's regime this week after repeated disturbances there.
The Metropolitan Police said they were called at about 0040 GMT and officers were providing assistance at the scene, near Heathrow airport. Fire crews are also on standby.
The disturbances had spread to all four wings of the centre.
A Home Office spokesman said specialist prison officers were drafted in when several fires were lit in the early hours, setting off the sprinkler system.
The centre opened in 2001
Lin Homer, head of the immigration and nationality directorate, said: "The situation is contained and the perimeter at Harmondsworth has not been breached."
Dozens of extra officers, including riot police, have been sent to the centre along with several police security vans and dog handlers.
Ms Homer would not confirm that officers sent to quell the disturbances had full control of all four wings.
The centre, which can hold up to 501 people, currently has 482 people staying there.
In a report published on Tuesday, chief inspector of prisons Anne Owers said the complex was hard to run, but her report was the poorest ever on a removal centre.
Detainees said they feared bullying, and staff were unaware of a special plan to prevent suicides.
The Home Office said it would draw up a plan to improve the centre.
In 2004 a detainee committed suicide, sparking a major disturbance that led to its temporary closure.
Since then, Harmondsworth has been at the centre of ongoing campaigns against detention of failed asylum seekers.