A plan to ease the overcrowding crisis in English and Welsh jails by moving convicted prisoners into police cells has come into force.
More than 500 police cells could eventually be used
The Home Office said 47 inmates were being housed in police cells under Operation Safeguard.
Home Secretary John Reid accepted the move was "necessary and pragmatic". But the Conservatives labelled it a "short-term and costly measure".
Nineteen police forces have made about 240 cells available.
The jail population has reached 79,714 - 65 below the all-time high of two weeks ago.
But using police cells became necessary last weekend because jails in some parts of the country have been more badly affected than others.
The Home Office spokeswoman said there was potential for 520 police cells to be used during October and November if they were needed.
"The number of cells has been agreed with chief constables, who have used their operational judgment as to how many cells they can staff and make available."
The measures would help to "manage the short-term pressures on the prison population" and would be kept under close review, she said.
But shadow home secretary David Davis said the measures would only buy the government a few weeks and would "place even more burdens on our police".
"We have only had to resort to it because of the government's utter failure to address the chronic lack of capacity in our prisons."
Operation Safeguard was last used in 2002, when 275 prisoners were placed in police cells as a result of an overcrowding crisis.
By then, the number of people imprisoned in England and Wales had reached 72,000.
The emergency measure can be used under The Imprisonment (Temporary Provisions) Act of 1980, which enables prisoners - sent to prison by the courts - to be held by police if there is no room for them to enter the prison system.
The move can only be activated by the Home Office - through the director general of the Prison Service.
Campaign group the Prison Reform Trust was also critical of the measures, accusing the home secretary of desperation.
Juliet Lyon, the charity's director, said: "Police stations are generally not equipped for visits, exercise or association.
"Using police cells to warehouse prisoners is a desperate measure with no pretence at decency or rehabilitation."
She said the trust was worried about the risks police officers faced in "trying to look after very vulnerable people with no space or resources to do so".
Under the system police cells are used mainly to contain adult male prisoners who have been remanded into custody to appear in court.
The most serious offenders (category A); those at risk of self-harm or escaping; women inmates; those under 18; people on trial at a crown court and anyone needing medical care are thought to be unlikely to be housed in police cells.
Liberal Democrat home affairs spokesman Nick Clegg said: "Prison cells are enormously expensive, and looking after prisoners will be a huge distraction for the police."