The walkout by prison officers in most jails is unlawful and ministers are considering court action to end the dispute, the Government has said.
Ministers could seek an injunction forcing staff at prisons across England, Wales and Scotland to return to work, the Ministry of Justice said.
Prison officers started unannounced protest meetings at 7am on Thursday 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, 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 the 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." 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.
The Public and Commercial Services union 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.