Prison officers have been threatened with legal action for holding protest meetings as up to 400,000 public sector employees, including police officers, lecturers and border control staff, stage a day of action against the government austerity programme and pension changes.

The Prison Officers Association (POA) said the meetings had been taking place at "80% of prisons" since staff began work on Thursday morning and would continue until they were called off by the union's national executive.

Steve Gillan, the POA general secretary, said the union had been warned by the Treasury solicitor that ministers considered the protest meetings to constitute industrial action and would seek an injunction if they continued.

The surprise action by prison officers, who are banned from striking, came as off-duty police officers and trade union leaders attended protests in central London. Public sector unions held their third one-day walkout in 12 months, led by the Public and Commercial Services (PCS) civil servants' union, health workers from the Unite union, teachers and lecturers from the University and College Union, the Nipsa civil servants' union in Northern Ireland and Royal Navy support staff at the RMT union.

The PCS expected around 200,000 civil servants to join the walkouts but the Cabinet Office said the number was closer to 100,000, fewer than the previous national walkout on 30 November, which was bolstered by civil service unions that have not taken part in Thursday's action.

"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," said Francis Maude, the cabinet secretary.

The focus of the protests was in London, where the Police Federation held a rally attended by an estimated 30,000 off-duty officers. Leaders of the PCS, Unite and RMT unions spoke at a rally in Westminster this afternoon.

One of the police marchers, Sergeant Al Perry of the West Yorkshire Police Federation, said: "We are here for one reason: to put our voice against the 20% cuts being imposed by the Home Office. There are around 20,000 officers here today. We want the public to look at us and see that this is the number of officers that will be lost in the next two years."

Yvette Cooper, the shadow home secretary, who joined the march, said police cuts would make it harder to reduce crime. "The government is going too far and too fast," she said.

Commenting on the police march, David Cameron's spokesman said: "The government inherited a very tough fiscal challenge. We are having to make spending cuts across the board. We think the reductions in spending on the police are challenging but manageable and that the police will still have the resources that they need to do the important work that they do."

The PCS strike included action by immigration officers at Britain's largest airport, but Heathrow's owner said there had been no significant disruption. A spokesman for BAA said there were no serious queues at Heathrow, apart from one hour-long wait for non-European Union passengers at terminal three.

One Thai Airways passenger told the Financial Times: "It was crowded in there." However, the BAA spokesman said passport control queues were hitting targets elsewhere at the airport. "Overall the Border Agency seems to have done a good job in getting its contingency resources in place."

Dominic Raab, the Conservative MP for Esher and Walton, said unions were hurting the hard-working majority by staging strikes. "Militant union leaders engaged in Soviet-style double-speak threaten Britain's economic recovery," he said.

The PCS general secretary, Mark Serwotka, said the strikes would show "the tide has turned" against public sector pension reforms.

The national strike by around 1 million public sector workers on 30 November was followed by a series of outline agreements for civil service, health, education and local government employees. Pension changes were included in the Queen's speech, despite opposition, including pegging the public sector pension age to the state pension age, and higher contributions.

Major health, civil service and education unions have not backed the changes, and Serwotka said he would urge the Trades Union Congress to reopen pension negotiations with the government.

Speaking before a rally in central London to mark the strikes, Serwotka said: "The significance of this strike is that the government clearly thought that 30 November was the end of it, and they are clearly trying to project the image that the issue is done. These strikes will show very clearly that this is not the case."