By Dominic Casciani
BBC News Online community affairs reporter
How does immigration detention work - and is violence at Harmondsworth an isolated incident?
Harmondsworth: Criticised in 2003
What is Harmondsworth for?
Harmondsworth is one of a string of immigration removal centres around the UK. The majority of people held in them are asylum seekers who have been told their cases have failed.
The latest available figures reveal that there were 370 detainees at Harmondsworth, 290 of these being asylum seekers.
Why are people held in these centres?
Removal centres hold people who are either in breach of immigration regulations or thought to be likely to "abscond" while waiting a decision on their case.
Harmondsworth also handles "fast-track" removals where officials believe someone's case is sufficiently clear cut to move to a quicker deportation.
But the most controversial aspect is those held have not actually been convicted of any crime
In theory, those being held at immigration removal centres have exhausted their legal rights to stay in the UK and are awaiting deportation. But in practice many removals can take months to carry out.
So how long are people held for?
Official figures reveal substantial disparities in how long people are held.
Of the 1,600 people in detention on 27 March 2004, 585 had been held for less than two weeks. But 130 people had been held for up to 12 months - and 80 for more than a year, their cases having not ended either way
LENGTH OF DETENTION
Less than 14 days: 585
Less than a month: 255
Up to four months: 80
Four to six months: 140
Six - 12 months: 130
More than a year: 80
Source: Home Office
There are no official statistics for the number of families being held but Home Office figures say there were 30 children being held in detention.
Campaigners complain one man still being detained at Harmondsworth has been there for 11 months without being given a deportation date.
The government says the weekly bill for each detainee ranges from just under £400 to £1,600, depending on where they are kept.
What does the government say about these centres?
Removal centres are a key plank of the government's asylum strategy. It has been expanding the detention estate, saying it is essential that those who should be deported are removed efficiently in order to maintain public confidence.
But Anne Owers, the Chief Inspector of Prisons, has not liked what she has found since she began sending teams into the institutions.
She has frequently criticised conditions, not least the quality of access to legal advice for detainees.
Her 2003 report on Harmondsworth described the centre as "unsafe" because of growing levels of disorder, damage and escape attempts.
One of her chief concerns about the system overall has been the holding of families. Ms Owers has told ministers that children should only be held in removal centres for the least possible amount of time prior to deportation.
If not, she warns, the welfare and health of the children is being compromised.
Why are families held?
Immigration officials can hold families for the same reasons that they can hold anyone else. But there has been a concerted campaign to get children out of centres, principally the special family unit at Dungavel removal centre in South Lanarkshire.
For its part, the government says the decision to detain families is not taken lightly and the average time children are now being held is 11 days.
A few weeks ago, Home Secretary David Blunkett said he had made good on this pledge by visiting Dungavel himself. He said he found conditions to be "entirely satisfactory" and praised "dedicated and caring staff".
So does everyone get removed?
No. There are many cases of people being held for months and then being released because they have won a bail hearing.
This is one of the major complaints about asylum detention.
Yarl's Wood: Fire following riot
While every detainee has a right to seek a bail hearing, campaigners say the advice available - including the quality of some of the duty lawyers - is not good enough for people to ensure they get a fair hearing.
At one centre, Lindholme in South Yorkshire, the prisons inspector found staff charging detainees for bail advice leaflets which had been freely provided by a charity.
What is the climate like in removal centres?
Few would describe the hard work of removal as a pleasant experience for either the detainees or staff. There have been a number of reports of low staff morale at various centres, especially those which have experienced suicides or attempted suicides.
Campaigners claim some removals have led to instances of abuse and physical assault.
Those who have recorded what goes on inside the centres say morale then drops further, triggering a rise in tensions.
A number of centres including Harmondsworth have experienced hunger strikes and other forms of non-co-operation with officials.
So the trouble at Harmondsworth is not the first such incident?
Not at all. The worst outbreak of violence happened in February 2002 when detainees rioted at Yarl's Wood detention centre, after a female asylum seeker was restrained.
Half of the flagship centre was completely destroyed in the subsequent fire, though the institution has been reopened.
This year, Prisons Ombudsman Stephen Shaw confirmed allegations of racism within the centre.