British unions say up to two million public sector workers joined a strike Wednesday over government plans to weaken their pension rights as part of its austerity programme. The action shut thousands of schools, paralysed local authorities and left hospitals to operate on minimum staffing, in what unions called the biggest walkout in decades. But the government contested the unions\' participation figures. London Metropolitan Police said the city\'s ambulance service had been forced to ask them for help responding to emergency calls across the capital due to a staffing shortage. However, fears of long delays at London\'s Heathrow airport, one of the world\'s busiest air passenger hubs, failed to materialise as most immigration officials turned up for work. Ferry ports and cross-Channel rail services linking Britain to continental Europe also operated largely as normal. And thousands of workers marched through central London and Manchester, northwest England, during the 24-hour strike. London police confirmed they had made 75 arrests for a variety of offences committed during the capital\'s demonstration. Birmingham, Britain\'s second city, in central England, hosted another major union rally and unions said 300,000 workers had stopped work in Scotland. The strike was the biggest test so far of Prime Minister David Cameron\'s Conservative-Liberal Democrat government. It was their plans to make public sector workers pay more into their pensions and work longer that sparked the unions\' fury. Under the government\'s proposals -- which form part of its efforts to slash the budget deficit -- public sector workers will have to work until they are 66 and increase the amount they pay into their pensions. But staff also face a lower pension payout, based on their average salary as opposed to the final salary schemes, to which they are currently tied. The unions\' anger rose even further on Tuesday when finance minister George Osborne announced a new two-year, one-percent cap on public sector pay rises and plans to cut an extra 300,000 public sector jobs. The announcement came as he announced a sharply reduced growth forecast for Britain. Unions expressed outrage at the plans, which would hit the pay of teachers, nurses and soldiers. But Cameron was scathing about the strike, telling parliament it had been a \"damp squib\" and lambasting unions for calling the action while negotiations on pensions were ongoing. The prime minister insisted that reforms to public sector pensions were \"absolutely essential\". And he accused Ed Miliband, leader of the opposition Labour Party, of being \"irresponsible, left-wing and weak\" for refusing to condemn the strike. Unison, which represents 1.3 million public sector workers, said two million workers had taken part in the stoppages, and they claimed they also had wider support. But Britain\'s Cabinet Office on Wednesday challenged this. \"The figures we have show turnout was much lower than these claims and significantly less than the unions predicted,\" it said in a statement. Brendan Barber, general secretary of the Trades Union Congress (TUC), the umbrella group for Britain\'s unions, said the government had put the public sector \"under attack\" and the strike was fully justified. \"There comes a time when people really have to stand up and make a stand,\" he told ITV television. \"With the scale of change the government are trying to force through, making people work much, much longer and get much, much less, that\'s the call people have made.\" Neil Clarke, a union organiser with Unite, another major public sector union, said: \"The government is attacking our pension schemes -- they are looking for public sector workers to contribute more, work longer and receive less in pension benefits. \"The average public sector pension comes in at £3,000 ($4,650, 3,500 euros) a year. Could you live on £3,000 a year?\" BAA, the operator of Heathrow Airport, said queues at border control points were \"normal\", despite earlier fears that there might be delays of up to three hours.