Thousands of UK schools closed on a day of strikes by public sector workers over pension changes.
But the government said action by civil servants had had "minimal" impact with fewer than 100,000 on strike at midday.
The Public and Commercial Services union said it was the best supported strike it had ever held, with 200,000 civil servants striking.
Jobcentres, border controls and passport offices were also affected by the industrial action.
Thousands of schools were closed across England and Wales on Thursday as teachers from three unions walked out.
The government says the proposed pension changes are "fair to taxpayers" and other unions are continuing with negotiations.
It condemned the strike, as did the opposition, although Labour leader Ed Miliband accused ministers of mishandling negotiations with the unions.
A Downing Street spokeswoman said: "Our border controls are in place and Jobcentres and pension offices are open for business.
"Indicative figures from every government department show that as of 12 noon today, over 75% of civil servants were not on strike. Just fewer than 100,000 civil servants were on strike - around one-fifth of the workforce.
"This shows that less than half of PCS members decided to take strike action today."
Mark Serwotka, leader of the Public and Commercial Services union, said up to four million workers could strike in the autumn if the bitter row is not resolved.
He also said 85% of his members had been on strike, MPs had refused to cross picket lines and staff in Downing Street had taken action.
"The government has been rumbled, and ministers are either badly briefed - or they are lying," he said.
The action by the National Union of Teachers (NUT), the Association of Teachers and Lecturers (ATL) and the University and College Union (UCU) affected England and Wales.
The unions say the proposals would mean more work and contributions for a reduced pension.
NUT General Secretary, Christine Blower, said on BBC's Question time: "Much of this has already been imposed on teachers without negotiation, and when we say there are talks going on it's perfectly true that the government is talking but it isn't actually listening.
"This is actually an affordable scheme, so the issue is not about affordability, it's about whether you want to afford it or not."
At least 12,000 schools in England and Wales are known to have been closed or partly closed. Unions say the total is even higher.
Department for Education data suggests that 11,114 of the 21,500 state schools in England were hit by the walkouts - the department based its calculations on data from 80% of schools.
It said in total, 5,679 schools were shut, and another 4,999 were partially closed. Some 201 academies and city technology colleges were also shut, while 235 remained partially open.
In Wales, according to local authority figures, more than 1,000 out of 1,800 schools were either closed or partially closed.
The NUT estimated 85% of schools in England and Wales were affected.
Kevin Courtney, the union's deputy general secretary, said the union realised the action was "very disruptive for parents," and said that "we do regret that".
He added: "We had hoped to reach a settlement before the industrial action, but the government isn't serious about talks."
The PCS also includes police support and border staff and some UK Border Agency staff walked out from 1800 BST on Wednesday.
However, airports and ports reported few difficulties.
Mark Serwotka, general secretary of the PCS, said his members were left with no choice but to take action as the government was not prepared to "compromise on any of the central issues of the strike".
"While they are talking, they are not negotiating," he told BBC Radio 4's Today programme.
The walkouts by the PCS, which has around 260,000 members, were staged across the UK.
- The Metropolitan Police Commissioner Sir Paul Stephenson says 90% of police staff who handle calls from the public had not turned up for work and as a result officers were responding more slowly to requests for help
- About 30,000 workers went on strike in Scotland, including staff at the Scottish parliament, the passport office in Glasgow and civilian employees at defence establishments such as HMS Naval Base Faslane
- In Wales, all national museums were closed to the public and the DVLA operated a reduced service
- Home Office employees were among 3,000 people striking in Northern Ireland. Belfast International Airport warned of possible flights delays as immigration and customs officers join the action
- There were PCS pickets outside the Old Bailey but the courts still opened. At Inner London Crown Court, there were no trials sitting with juries and the PCS union says other Crown, magistrates and coroners' courts were disrupted
- The Maritime and Coastguard Agency said there have been some walkouts but all coastguard stations are "operational and appropriately manned"
- The Department of Work and Pensions said 18 of the 750 Jobcentre Plus offices across the country were closed, and 40% of staff were on strike
- In London, where police leave has been cancelled, union leaders and activists marched to Westminster for a rally
- In Liverpool, some 537 out of the 550 staff at the passport office were on strike. Most civil servants at Ministry of Defence and Criminal Records Bureau offices in the city also walked out
Cabinet Office Minister Francis Maude said: "What today has shown is that the vast majority of hard-working public sector employees do not support today's premature strike...
"Reform of public sector pensions is inevitable, but we will ensure that public sector pensions will still be among the very best, with a guaranteed pension which very few private sector staff now enjoy. But they will be paid later because people live longer."
Writing on Twitter, Mr Miliband said: "These strikes are wrong at a time when negotiations are going on. People have been let down by both sides - the Govt has acted recklessly".
Some striking workers spoke of their "anger" at Mr Miliband's refusal to back their walkout. PCS union members on the picket line outside the House of Commons said the party should stand up for their rights.