The walkout by prison officers in most jails is unlawful and ministers are considering court action to end the dispute, the Government said today.
Ministers could seek an injunction forcing staff at prisons across England, Wales and Scotland to return to work, the Ministry of Justice (MoJ) said.
Prison officers started unannounced protest meetings at 7am against Government plans to link their normal pension age to the state pension age.
Members of the Prison Officers Association (POA) are taking limited action at the majority of jails, prison sources said.
Michael Spurr, chief executive of the National Offender Management Service (Noms), said: "I am extremely disappointed that the POA has taken this unlawful action.
"We have implemented our contingency plans, and our priority is to protect the public and ensure that prisons remain safe and secure.
"In 2007, the POA agreed that the normal pension age for new prison officers would be 65, in line with all other civil servants.
"The Government has been in constructive discussions with the POA about further pension reform and it is deeply regrettable that this action has been taken now."
Asked if the Government could seek the injunction to force staff to return to work, a Ministry of Justice spokeswoman said: "Ministers are keeping all options for bringing this action to an end under consideration."
It comes as union leaders predicted that up to 400,000 workers, ranging from police officers and immigration staff to lecturers and job advisers, will be involved in a wave of demonstrations.
The row was fuelled by ministers making clear in yesterday's Queen's Speech that they are pressing ahead with their controversial reforms.
Steve Gillan, the POA's general secretary, said: "The POA has submitted a case to Government to support our view that it is unrealistic for prison officers to be automatically linked to the state pension age, which will ultimately rise to 68 years of age.
"Unfortunately, it has fallen on deaf ears and prison officers have no other option but to protest to gain public attention."
POA chairman PJ McParlin said: "We are an essential uniformed service in a volatile operational workplace. A pension age of 68 is unacceptable to this trade union. We will protect our pensions. We have a right to retire from service not to die in service."
The union added that branch officials have been briefed to ensure that minimum cover arrangements are in place to ensure prisoner safety.
Cabinet Office minister Francis Maude described the strike as "futile" and insisted that talks over pensions will not be reopened.
Prison staff take part in a protest outside HMP Manchester
The Public and Commercial Services union (PCS) said early signs from picket lines showed solid support for the strike, the third major walkout by public sector employees in the past six months in protest at the pension reforms.
Recorded messages on HM Revenue and Customs phones were advising people to call back another day, and there were reports of government offices and jobcentres being closed, said the PCS.
A spokesman said there was "very strong" support among Border Agency staff at all ports and airports, while in London, 999 call staff and community support workers were out in big numbers.
"Investigators from the Serious Organised Crime Agency are also showing strong support because they are keen to show their solidarity for police colleagues marching today," said an official.
Passengers arriving at Gatwick Airport were warned they may experience some delays at immigration, but a Border Force spokesman said a "trained pool of contingency staff" were being used to minimise disruption.
"Thanks to our preparations, delays are being kept to a minimum and we will continue to deploy staff to manage peak arrivals during the day," he said.
Maude said: "It is very disappointing that a handful of unions insist on carrying on with futile strike action which will benefit no one.
"We would urge these union leaders to reconsider their position. Pension talks will not be reopened and nothing further will be achieved through strike action."
Meanwhile, up to 16,000 off-duty officers, wearing black caps representing each officer expected to be lost under the Government's budget cuts, took to the streets of the capital.
The officers, banned from striking under law, are marching through central London in a protest against proposed changes to their pay and conditions.
Some 20,000 officers from all 43 forces across England and Wales were expected to take part in the first police march in the capital for more than four years, organisers said.
Jails across the UK were also hit by strikes according to prison officers outside the gates at HMP Manchester, who said colleagues had come out at most prisons.
Staff, who like police officers are not legally allowed to go on strike, called their action a "protest" after 100 members on the 7am shift left their posts to stand outside the main gate following a meeting with representatives from their union, the Prison Officers Association (POA).
Some staff have agreed to stay inside to ensure the Category A jail's 1,200 prisoners are fed and given medication.
Prison Officer Mike Lowe, 41, from Merseyside, said members were angry that their retirement age of 60, which was in line with police, army and fire service, had now been raised to 67 by the Coalition government.
Prison Officers will also face mandatory fitness tests to ensure they can do the job despite advancing years.
Lowe said: "How would the general public like to think about their grandad or grandma, aged 67, being abused or having to tackle a violent prisoner?"
"How can the Government expect someone at that age to run up four flights of stairs and restrain a violent prisoner in his 20s?"
"Prison officers are angry, we do a tough job and all we are asking for is a fair deal."
Prison staff said it was only the second time in their history they had held a strike.
Another prison officer, who did not want to be named said: "At Strangeways we have got some of the most dangerous prisoners in the country - murderers, terrorists, gangsters, psychopaths who are here because there are no beds in mental institutions.
"Staffing levels are already horrendous and there are people being assaulted on a daily basis. Would you want your 67-year-old grandad to have to tackle a 20 year-old con who spends all his time in the gym?
"We invited Ken Clarke (Justice Secretary) to try to do the fitness test - he declined.
"He couldn't pass when he is 23 never mind 63 - he's too well fed."
Brian Lord, HMP Manchester branch secretary for the POA, added: "We feel, as a unit, raising the retirement age to 67 for us is unsafe.
"People will be dying in service. It's just not do-able."
Lord said the pay of a Prison Officer ranges from £21,000 to £28,000, though cuts mean this will fall to a maximum of £26,0000.
He said the maximum annual pension a prison officer could get, after 30 years' service, would be around £14,000 per year.
Juliet Lyon, director of the Prison Reform Trust, said: "We recognise valid concerns raised by the POA and, at the same time, fear that unannounced action of this kind is bound to have a damaging impact on people in prison and their families ranging from lock-downs to cancelled visits."
Maude said later: "The dedicated majority of public sector workers are working normally today and rigorous contingency plans are ensuring that nearly all key public services remain open as usual.
"We can now confirm that far fewer civil servants are on strike than in November - with around 100,000 taking part - down from 146,000 last year. This is dramatically lower than union claims."