Hundreds of thousands of public sector workers have gone on strike across the UK over planned pension changes.
Teachers from three unions walked out, with at least 40% of state schools in England and Wales disrupted.
Workers in numerous government departments and agencies also did not turn up for work, and their union said turnout was strong throughout the UK.
However, the government disputed union claims, saying fewer than 100,000 civil servants were on strike at midday.
The government said the public sector pension plans were "fair to taxpayers" and the other 25 public sector unions not on strike on Thursday were continuing with negotiations.
The Public and Commercial Services union said early indications from pickets suggested about 210,000 of its members participated in the strike.
However, the government said figures gathered "from every government department" indicated that just under 100,000 civil servants went on strike, meaning 75% were at work.
With three teachers' unions taking action, the government said about 40% of state schools in England and Wales have been closed or partially shut.
It has condemned the strike as has the opposition, although Labour leader Ed Miliband has accused ministers of mishandling negotiations with the unions.
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.
Department for Education data suggests that 11,114 of the 21,500 state schools in England were hit by the walkouts.
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, 1,100 out of 1,800 schools were either closed or partially closed.
Both the ATL and NUT said the figure was a lot higher - with 85% disrupted
The impact of the co-ordinated industrial action began to be felt at ports and airports on Wednesday evening, when some UK Border Agency staff walked out from 1800 BST.
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".
He said he hoped the government would "change direction and engage with us, but if they don't the unions are talking about further action like this in the autumn".
The walkouts by the PCS, which has around 285,000 members, were staged across the UK.
- The Metropolitan Police Commissioner Sir Paul Stephenson said 90% of police staff who handle calls from the public have 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 were open. At Inner London Crown Court, there were no trials sitting with juries and the PCS said other Crown, magistrates and coroners courts were disrupted
- The Maritime and Coastguard Agency said there were some walkouts but all coastguard stations were "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 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 were also walked out
There were 30 arrests in London in connection with the march, including 24 people arrested for a variety of offences including possession of drugs, criminal damage and breach of the peace. They remain in custody at various London police stations, while six were released.
One police officer and six members of the public were injured during the London march.
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."
Earlier, he told BBC One's Breakfast: "People are going to be scratching their heads, wondering why teachers and some civil servants are going on strike while discussions are still going."
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".
Meanwhile, Kevin Courtney, deputy general secretary of the NUT, said: "We realise that it's very disruptive for parents and we do regret that. We had hoped to reach a settlement before the industrial action, but the government isn't serious about talks."
Miles Templeman, director general of the Institute of Directors told the BBC he believes the strikes have the potential to damage the UK economy.
He said: "We want to be seen as the best country for people to invest in - these sort of messages send something very different.
"The private sector has had to wake up to the tough realities of pension provision in a rapidly changing world, and the public sector must do the same."
BBC political editor Nick Robinson said that people on both sides of the negotiations, as well as Labour Party figures with good union connections, "continue to believe that a deal is the only way that will see the resolution of this - and that a deal is possible".
TUC general secretary Brendan Barber told a rally in Exeter: "The living standards of low and medium-paid public sector workers are being hammered in the name of reducing the deficit... Meanwhile those who caused the crisis are getting off scot free."