Up to two million public sector workers are staging a strike over pensions in what is set to be the biggest walkout for a generation, according to BBC.
Schools, hospitals, airports, ports and government offices will be among sites disrupted, as more than 1,000 demonstrations are due across the UK.
The chancellor urged more talks, saying strikes would not achieve anything.
Unions object to government plans to make their members pay more and work longer to earn their pensions.
The strike has impacted:
- About 2,700 schools out of 21,700 are open in England says the BBC`s education correspondent Gillian Hargreaves.
- The Department of Education says it is expecting 13% of state-funded schools in England, including academies and free schools to open and 13% to be partially open. The status of 16% of schools is unknown.
- Plane arrivals and take-offs at Britain`s two biggest airports - Heathrow and Gatwick - are largely unaffected with only a few cancellations of inbound transatlantic flights to Heathrow.
- Heathrow operator BAA, and its busiest carrier, British Airways, have both reported near-normal services, with queues at immigration no longer than usual.
- At Heathrow, the 10 EU UK desks at Terminal 3 immigration are being manned by mix of home office staff and police officers who have been trained. Five non EU desks are open as airport sources suggest immigration controls are at two thirds of normal staffing levels
- In Northern Ireland, no bus or train services will be operating and two thirds of schools and colleges will be closed.
- About 300,000 public sector workers are set to go on strike in Scotland while 170,000 workers in Wales are to take action
Speaking from Brussels, Chancellor George Osborne told BBC Breakfast: "The strike is not going to achieve anything, it`s not going to change anything. It is only going to make our economy weaker and potentially cost jobs.
"So let`s get back round the negotiating table, let`s get a pension deal that is fair to the public sector, that gives decent pensions for many, many decades to come but which this county can also afford and our taxpayers can afford.
"That is what we should be doing today, not seeing these strikes."
He added that without making difficult choices about dealing with the UK`s debts the country "would be bankrupt".
Labour leader Ed Miliband said he had "huge sympathy" for people whose lives are disrupted by the strike.
But he said he was "not going to condemn the dinner ladies, nurses, teachers who have made the decision to go on strike because they feel they have been put in an impossible position by a government that has refused to negotiate properly".
Shadow chancellor Ed Balls told BBC Breakfast the pensions row should have been resolved by the government.
On Tuesday night, shadow chief secretary of the treasury, Rachel Reeves, told BBC`s Newsnight that Labour did not support the industrial action.
"We do not support the strike because a strike is a sign of failure," she said.
Earlier on Tuesday, union leaders reacted angrily to Mr Osborne`s Autumn Statement announcements of a public sector pay cap of 1% for two years, as well as bringing forward to 2026 the rise in the state pension age to 67.
`Failure to negotiate`Unison general secretary Dave Prentis told BBC Radio 4`s Today Programme that industrial action by his union was rare but public sector workers "were annoyed". Millions of workers - mainly low-paid women - were being unfairly affected by changes to pensions, he said.
"That is when people say enough is enough," he added.
Brendan Barber, general secretary of the TUC, said the public sector was "under attack" by the government, adding that the action was justified.
"There comes a time when people really have to stand up and make a stand," he told ITV`s Daybreak.
"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."
Paul Noon, leader of civil service union Prospect, said members felt the chancellor was "aiming yet another punch at them".
The 24-hour strike is expected to disrupt courts, job centres, driving tests and council services, such as libraries, community centres and refuse collections. Highways Agency staff will be on strike, as will many Police Community Support Officers (PCSOs).
Education Secretary Michael Gove has said it is "unfair and unrealistic" to expect taxpayers to foot the growing public sector pensions bill.
Schools Minister Nick Gibb told BBC Radio 4`s Today Programme: "We do understand people are concerned about pensions and we are determined to maintain defined benefit pensions. The negotiations are continuing.
"We have to be fair to all taxpayers not just those who work in the public sector. If we`re going to sustain these kind of high quality pensions in the long run there does have to be reform."
General secretary of the National Association of Head Teachers Russell Hobby responded that "blame for any rise in union militancy - particularly among moderate unions - belongs fairly and squarely at the government`s door: A failure to negotiate in any meaningful sense until the last minute".
The UK Border Agency is set to be hit by the walkout of Public and Commercial Services (PCS) union members, and the agency has warned that "people travelling into the UK may experience delays at border control".
Airport sources suggested to the BBC that immigration controls are at two thirds of normal staffing levels - more than the 30-50% predicted previously.
The government has said no border controls will be relaxed to ease queues.
Hospital managers are planning to postpone thousands of non-emergency operations because of the strike.
Patients needing urgent treatment such as chemotherapy and kidney dialysis will still be able to get it, and maternity units will remain open.
Calls to 999 will still be answered, but patients are being urged to think carefully and call only if it is a genuine emergency.
The strike was "irresponsible and reckless", said John Longworth, the director general of the British Chambers of Commerce.
He added: "Trade unions are living in a bubble and ignoring the fact that Britain has to make its way in a competitive world."
Weather forecasting staff at the Met Office, catering staff in the House of Commons and museum curators are also among those due to walk out on Wednesday.