A group of people at an immigration removal centre in west London caused a "disturbance" on Friday night following a power outage, the Home Office has said.
No one was injured during the incident at Harmondsworth detention centre near Heathrow Airport, but the power was still out just before 9am on Saturday, the department said.
A group of detainees left their rooms and went out into the courtyard area armed with various weaponry, according to reports.
Police, fire and HM Prison services are at the scene.
A Home Office spokesperson added: "There has been a power outage at Harmondsworth immigration removal centre, and work is currently under way to resolve this issue.
"We are aware of a disturbance at the centre and the appropriate authorities have been notified and are on scene.
"The welfare and safety of staff and individuals detained at Harmondsworth is our key priority."
In a statement Mitie, which operates the site, said the power outage occurred during a power cut in the local area.
It said: "We are working closely with the Home Office to ensure the safety of those in our care while work is carried out on site to resolve the issue."
Electricity supplier SSE said in statement: "We experienced a high-voltage underground cable fault at midnight on Thursday which affected electricity supplies to 290 customers for one minute (including Harmondsworth).
"We are not aware of any issues with the electricity distribution system in that area since."