London (CNN) -- Hundreds of thousands of British teachers, air traffic controllers, customs officers and other public sector workers went on strike Thursday, causing disruption to schoolchildren and travelers.
Workers demonstrated in many British cities, including London, where thousands of strikers marched peacefully in the center of the city, their route taking them near the prime minister's office at 10 Downing Street.
"We've paid into our pensions, we've paid our taxes," striking adult education tutor Annie Holder said, adding that she was "really angry about the government's politically motivated attempt to steal our pensions."
She blamed "the banking sector" for the country's budget woes.
And she rejected rhetoric from opponents of the strike about the public sector's "gold-plated pensions."
"Our pension will be about 60 pounds ($96) a week. It's hardly gold-plated. We'll have to work much harder and pay more," Holder said.
The government's Cabinet Office played down the level of support for the strike, saying essential public services had stayed "up and running" on Thursday and that most civil servants had reported for work.
Police in London said they had made 24 arrests in total as of mid-afternoon.
Since Thursday morning, 18 had been arrested for offenses including possession of drugs, criminal damage and breach of the peace, the police said, with six others detained overnight in Trafalgar Square.
Police declined to estimate the size of the crowd, but the Public and Commercial Services union said about 15,000 people rallied in London.
CNN reporters in the central Whitehall area said there were more police and media present than protesters, and that there were minor scuffles at one point. The police closed off public access to Trafalgar Square for a period.
An estimated 5,000 people marched in Manchester, 4,000 in Brighton, and 1,000 in Cardiff and Glasgow, the PCS said.
Four unions told their members to stop work over planned government changes to the pension system.
Perhaps ironically, state pension staff are among those on strike, as members of the PCS.
Three teachers' unions are also on strike -- the National Union of Teachers, the Association of Teachers and Lecturers, and the University and College Union -- which together have more than 350,000 members.
The PCS, Britain's fifth biggest union, boasted it had 84% participation from its 300,000 members.
But the government's Cabinet Office said its figures showed that as of Thursday afternoon, 105,890 civil servants were on strike, meaning that only 42% of PCS union members had walked out.
The Cabinet Office said about 80% of the civil service workforce was not on strike and that the numbers taking part in the action were lower than in previous strikes called in 2004 and 2007.
The PCS union's general secretary, Mark Serwotka, who spoke at a rally flanked by leaders of the teachers' unions, said: "Our members have voted with their feet and supported the strike. We are in it together with public sector workers, students and pensioners defending everything we have fought for for generations."
Some 80% of schools across the country were closed or partially closed as a result of the strike, the National Union of Teachers said, and there were fears that airports and ports would be snarled as well.
Nine out of 10 police staff who answer calls from the public were on strike, London's Metropolitan Police said.
The National Union of Teachers said the strike is because "the government is planning to cut your pension. They want you to pay more, work longer and get less," arguing that because pensions are "deferred pay ... you are effectively being asked to take a pay cut."
The government, a coalition of Conservatives and Liberal Democrats, is trying desperately to slash government spending in the face of huge deficits.
Danny Alexander, the No. 2 official in the British Treasury, argued earlier this month that "it is unjustifiable that other taxpayers should work longer and pay more tax so public service workers can retire earlier and get more than them."
"It is the employees who are benefiting from longer life and generous pensions, but it is the taxpayer who is picking up the tab," he said.
Alexander, a Liberal Democrat, said the changes the government was proposing aimed to ensure that "public service workers continue to receive among the best, if not the best, pensions available."
Holder, the striking teacher, said the government's explanations for planned changes to the pension system were "nonsense."
Cabinet Office Minister Francis Maude, a Conservative, said Wednesday that the strike was "premature" while negotiations between the government and unions were still going on.
He argued that only a minority of civil servants and teachers had voted to strike.
Feelings were strong, however, among many of the workers involved.
Jenny Adams, a teacher from Croydon, said, "We've got a situation where young people are not going to want to stay in this profession.
"It's about who's being asked to foot the bill for a mess that was made by others. We're in a profession that is not kind when it comes to age. It's inconceivable to be in a classroom in (one's) late 60s."
Union leader Dave Prentis warned last week that if the government does not change course on pension reform, the country could face the biggest strikes since 1926. Between 1.5 million and 1.75 million workers participated in a general strike lasting nine days that year.
Prentis, the head of Britain's largest public-sector union, Unison, issued a similar warning in 2005.
Unison is not participating in Thursday's strike but has not ruled out holding one in the autumn if the government presses ahead with its plans.
CNN's Dan Rivers, Jonathan Wald, Antonia Mortensen and Per Nyberg contributed to this report.