London (CNN) -- Mass strikes swept across the United Kingdom Wednesday, with public sector workers walking off jobs in schools, hospitals and police stations to protest proposed pension reforms.
The government said the majority of key public services remained open, however, although more than half of the country's schools were closed, affecting many families.
The unions said up to 2 million public sector workers could go on strike but early indications suggested the total might be fewer.
The proposed pension reforms have prompted wide anger among public sector workers, many of whom say they are bearing the brunt of austerity measures imposed to try to rein in Britain's deficit.
But Cabinet Minster Francis Maude played down the impact of the industrial action Wednesday evening, saying: "Despite the best efforts of union bosses the United Kingdom remains open for business."
Those traveling to Britain had experienced minimal disruption to their journeys as a result of border control staff striking, he said.
Some 146,000 civil servants joined the action, less than a third of the total, Maude said. The number on strike in the National Health Service was also lower than anticipated he said, at about 79,000, or 14.5% of the workforce.
About a third of local government staff in England and Wales did not show up to work, amounting to about 670,000 out of a workforce of 2.1 million, he said.
Good planning had limited the impact of the strike significantly, he said, adding: "Today's strike was inappropriate, untimely and irresponsible, especially while talks are continuing. Responsibility for any disruption today lies squarely with union leaders."
Prime Minister David Cameron earlier dismissed the industrial action as a "damp squib" -- a fizzled firecracker -- as he answered questions in Parliament.
"These strikes are wrong at a time when negotiations are going on," he said.
Unite, the country's biggest trade union, said strikes and marches were taking place in cities across the country.
London's Metropolitan Police reported 75 arrests connected to strike protests, about half of them in the east of the capital.
Dozens of Occupy London protesters stormed the offices of a mining company, Xstrata, in central London, saying the action was "in support of all those striking for fair pensions for all today." In a statement, they pointed the finger at corporate greed, saying a few are profiting while ordinary workers suffer.
About 20 Occupy protesters made it to the roof of the building, where they unfurled a banner on the side reading, "Power to the People." The Metropolitan Police said 21 arrests had been made at the site but that the incident was not related to the capital's pensions march and rally, "which passed off peacefully."
The Conservative Party, which governs in a coalition, is committed to a major austerity program of tax hikes and spending cuts, against a backdrop of rising unemployment, falling household income and a large public debt.
Chancellor George Osborne, the British finance minister, downgraded Britain's growth forecast Tuesday, saying that while it had regained stability after the financial crisis, it had been "hit by a series of shocks which have significantly weakened the economic and fiscal outlook."
And Cameron insisted the proposed pension reforms are necessary. "We have a responsibility to deliver an affordable public sector pension system," he said in Parliament Wednesday.
"We believe public sector pensions should be generous but as people live longer it's only right and only fair that you should make greater contributions."
Cameron criticized the opposition Labour Party over its attitude toward the strike, charging that Labour leader Ed Miliband "is in the pocket of the union leaders" and so cannot recognize a "very generous offer" to public sector workers.
Miliband said he had huge sympathy for those who had walked out because the government had put them in an "impossible position" by failing to negotiate properly.
Union leaders say it is unfair to expect public sector workers to pay more into their pensions, work longer and accept worse terms for their pensions.
Brendan Barber, general secretary of the Trades Union Congress, said the unions were "sending a crystal clear message to the government that we are strong, that we are united, and that our campaign will go on until we secure justice and fairness for every public servant."
No one takes industrial action lightly, he said, "but when unfairness is piled on injustice you are right to take a stand and I am proud to stand with every single one of you."
Chaos was predicted at Heathrow airport near London, one of the world's busiest international airports, but as of Wednesday evening operations were normal, the government said.
BAA, the company that operates Heathrow, said lines at immigration counters were moving smoothly thanks to contingency planning to mitigate the effect of strike action by border control staff.
British Airways said it had not canceled any of its own flights, although a few code-sharing flights were called off. BAA could not say how many flights were being canceled. Gatwick airport was reported to be operating normally.
The Unison union said it expected its 400,000 National Health Service workers -- including nurses, paramedics, cleaners, cooks and patient transport -- to strike Wednesday, except for emergency workers.
The union said it did not expect emergency services to be affected, but some routine operations and clinics were canceled.
The Home Office said "robust arrangements" were in place at airports and points of entry to compensate for the strike, with managers, contractors and foreign staff trained up to fill the gap.
The strike action came a day after Osborne, the Chancellor, announced a pay raise cap of 1% for public sector workers for two years after the current pay freeze ends, as he lowered growth forecasts in an assessment of the economy.
His report blamed higher-than-expected inflation, the uncertainty created by the crisis in the eurozone and the impact of the 2008-2009 financial crisis, the full scale of which is now clearer.
The Labour Party argues that the poor growth is a result of the government's spending cuts and tax hikes, which it says have squeezed the recovery too hard.
CNN's Jim Boulden, Erin McLaughlin, Laura Smith-Spark, Jack Maddox and Faye Clark contributed to this report.