Hundreds of thousands of public sector workers went on strike in Britain on Thursday in protest at pension reforms, planning to disrupt schools and airports in a major challenge to the year-old government. Four education and civil service unions have called out about 600,000 members in response to reforms they say will force workers to pay more and work longer to receive a reduced pension. Prime Minister David Cameron has said the strike is premature as the changes were still being negotiated, and warned there was no alternative to reform because \"the pension system is in danger of going broke\". Union leaders claimed a strong start to the one-day strike, predicting that about 80 percent of schools in England and Wales would be affected. Picket lines were also set up outside government buildings, job centres and law courts. \"We expect there to be hundreds and hundreds of thousands of workers out on strike,\" Mark Serwotka, the head of the Public and Commercial Services (PCS) union, told BBC radio. Among the PCS members due to walk out were thousands of immigration and customs staff. The UK Border Agency warned that ports, airports and railway stations would be affected, while airport operator BAA also warned of delays. However, Cabinet Office minister Francis Maude, a senior member of the Conservative-Liberal Democrat coalition government which took office in May last year, said fewer people were turning out than the unions had claimed. \"All the early indications I\'m getting from airports and ports are that fewer of his members are heeding his (Serwotka\'s) inflamed calls for action, more are turning up for work and we are maintaining a much better service than we expected to be able to,\" he said in a heated exchange with the union leader on BBC radio. Maude said the strike was premature given that negotiations on pensions were ongoing, noting that Unison, the biggest public sector union with 1.3 million members, had opted to keep talking, although it may strike later this year. He added: \"You cannot continue to have more and more people in retirement being supported by fewer and fewer people in work. Long-term reform is needed.\" But Serwotka accused ministers of refusing to compromise on their plans to raise the retirement age by up to six years, increase employee contributions and replace a final salary pension with one based on an average career salary. In a jibe at the several millionaires in the government, the union leader said the cabinet was full of people in a \"very privileged position\" trying to cut ordinary workers\' pensions. \"It is absolutely unfair and it is unjust,\" Serwotka said. The Conservative-led coalition has been the focal point of public sector anger after it announced a two-year pay freeze and 330,000 job losses by 2015 in an attempt to rein in a record budget deficit. About 250,000 members of the PCS were due to strike alongside about 220,000 members of the National Union of Teachers (NUT) and 55,000 from the University and College Union (UCU). A further 78,000 teachers and lecturers were called out from the ATL union, in the first nationwide strike in its 127-year history. Ed Miliband, the leader of the main opposition Labour party which has historically close ties to the trade union movement, has criticised the strikes as \"a mistake\" and \"a sign of failure\". A ComRes poll this week found 49 percent of the public believed the workers had a legitimate reason to strike, and 46 percent said ministers would be wrong to change public sector pensions if most workers affected opposed them. Large-scale industrial action is rare in Britain, which loses around half as many days to strikes as the EU average. Union activity is highly restricted, a legacy of reforms introduced by Tory premier Margaret Thatcher in the 1980s.