The Metropolitan police has had to send riot detainees to neighbouring forces after it ran out of cells, prompting unions and charities to claim the courts, probation services and jails are at "crunch point".

With more than 600 arrests and public pressure building for a hardline response, the Ministry of Justice said those held will appear in court "within hours" under fast-track procedures.

However, the probation union Napo said severe delays in cases are likely as courts and probation offices closed and sent staff home early. Community service schemes were also cancelled. Damage caused to the magistrates court in Tottenham, north London, meant all cases that would usually be heard there were being transferred to Highbury Corner, further south.

Harry Fletcher, Napo's assistant general secretary, said: "The administration of probation and courts in London is in chaos because of the disturbances.

"Courts, both crown and magistrates, and probation offices are shutting down early and this is likely to continue for the rest of this week. This will lead to severe delays in the administration of justice. Offender supervision will be cancelled and other projects severely curtailed."

However, the crown prosecution service said it had set up a "dedicated team ... of prosecutors available to provide charging decisions 24/7 as quickly as possible". Alison Saunders, chief crown prosecutor for London, said the service would "expedite cases arising from the disorder, and have established designated magistrates courts to deal with them".

But if the arrests lead to successful prosecutions, experts have warned sending hundreds of under-18s to jail would be an "unprecedented" test for the prison system at a time when custody levels in young offender institutions are above safe limits.

Ministry of Justice figures last month showed occupancy rates for such institutions above 94% – over the Youth Justice Board's safe operating limit of 93%. Experts said that the majority of the arrests were likely to be children and the figure could rise over the coming weeks as specialist teams of police trawl through hours of closed circuit television footage in riot-affected areas.

With 2,200 youths in custody, even if half of those arrested so far turned out to be aged 18 or under it would mean a 15% rise in the numbers of teenage prisoners.

John Drew, the head of the Youth Justice Board, which oversees the youth justice system in England and Wales, attempted to allay fears that the upsurge in violence would mean youth jails were unable to cope. He told CYP Now, a trade magazine focusing on youth services, 300 beds were available for young offenders: "We are more than able to deal with any upsurge in demand."

But Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said this was "not reassuring given the level of arrests".

He added: "We are seeing this all over the country and will need more than 300 beds. I think it's a crunch point for the ([prison)] system.

Prisons minister Crispin Blunt said: "A number of prisons and courts are being kept open on a 24-hour basis to deal with people arrested and charged over these disturbances. The criminal justice system will provide all necessary support to the police to ensure anyone involved in this kind of criminal activity is brought to justice as swiftly as possible.""

Neilson said one of the big issues already plaguing young offenders institutions was that teenage gang members from different parts of London ended up in the same prisons or wings.

The prison officers' association said it would do "all it could" to ensure that the influx of prisoners did not affect jail life, saying that there would inevitably be some mixing of "18-year-olds" and adult inmates which was not "good practice".

"Juveniles will be kept in separate wings. We are seeking a meeting with ministers about the situation. Our main concern is that there are no copycat disturbances in prisons as we have 85,000 people in prison and they don't need an awful lot to start up. Especially if we are moving them to make space," said the association.

• This article was updated on 10 August 2011 to clarify points made by Andrew Neilson