document: Some detainees began a hunger strike at Harmondsworth Detention Centre on Sunday in protest at living conditions.
  Earlier, they were filmed protesting in the exercise yard by aerial cameras.
  The Home Office said detention and removal were "essential parts of effective immigration controls" and it took detainees' welfare very seriously.
  The detention facility in West Drayton holds 615 men who are awaiting removal or deportation from the UK.
  A Home Office spokesman said: "Detention is only ever used as a last resort after all attempts to encourage individuals to leave voluntarily have failed."
summary: About 100 men at the UK's largest detention centre have been spotted in the exercise yard and are believed to be protesting about living conditions.