The Government's controversial plans to reform public sector pensions are a "good deal", the Prime Minister insisted as he told hundreds of thousands of teachers, lecturers and civil servants they are wrong to go on strike.
David Cameron said the changes being proposed for millions of public sector workers would secure affordable pensions for decades to come.
And Labour leader Ed Miliband has urged unions to continue negotiating with the Government, saying that strikes were a "failure".
Writing on his blog, he said: "I understand why teachers are so angry with the Government. But I urge them to think about whether causing disruption in the classroom will help people understand their arguments.
"You do not win public backing for an argument about pensions by inconveniencing the public - especially not while negotiations are ongoing."
They spoke out as the scale of disruption caused by Thursday's 24-hour walkout by members of four unions became clear, with thousands of schools, jobcentres, tax offices and courts set to be closed or badly disrupted.
Driving tests will be cancelled and customs checks at ports affected, while picket lines will be mounted outside Government departments.
Mr Cameron, addressing the annual conference of the Local Government Group in Birmingham, said reform was essential, warning that the pension system was in danger of "going broke" unless action was taken because people were living much longer.
"We just can't go on as we are. That's not because, as some people say, public service pensions are ridiculously generous. In fact, around half of public service pensioners receive less than £6,000 a year.
"The reason we can't go on as we are is because as the baby boomers retire - and thankfully live longer - the pension system is in danger of going broke."