Firefighters across England and Wales walked out on strike today in a bitter row over pensions, with the threat of further action if the dispute is not resolved.
Members of the Fire Brigades Union (FBU) left their stations and set up picket lines, leaving brigades to put into place contingency plans.
Some, including London and Surrey, were using private contractors to cover for the strikers, while others were relying on retained firefighters and volunteers.
The union has not ruled out further industrial action if the dispute continues.
A war of words was raging between the Government and the FBU as the strike went ahead.
The union is campaigning against changes it says will mean firefighters will have to work longer, pay more into their pensions and receive less in retirement.
The move will also see firefighters having to work on frontline duties until they are 60, the union argued.
But the Government maintained that the changes were fair and would still give firemen and women one of the most generous pensions in the public sector.
Fire minister Brandon Lewis said the dispute was over pensions, not complaints that firefighters will have to work on the front line until they are 60.
He said: "The Government has listened to union concerns - firefighters will still get one of the most generous pension schemes in the public sector.
"A firefighter who earns £29,000, and retires after a full career aged 60, will get a £19,000-a-year pension, rising to £26,000 with the state pension. An equivalent private sector pension pot would be worth over half a million pounds and require firefighters to contribute twice as much.
"The firefighter pension age of 60 was introduced in 2006 and is in line with the police and armed forces.
"We have been clear with the Fire Brigades Union our pension reforms are not introducing a national fitness standard. Firefighter fitness remains a local fire and rescue authority matter - Government is helping local employers and the union to work together on this issue."
FBU general secretary Matt Wrack said: "This initial strike is a warning shot to Government. Firefighters could not be more serious about protecting public safety and ensuring fair pensions. Governments in Westminster and Cardiff have simply refused to see sense on these issues.
"It is ludicrous to expect firefighters to fight fires and rescue families in their late 50s. The lives of the general public and firefighters themselves will be endangered.
"None of us want a strike, but we cannot compromise on public and firefighter safety."
TUC general secretary Frances O'Grady, who will join a picket line in Brighton, said: "There are many reasons why the Government shouldn't be pressing ahead with these unfair and ill-thought-out changes, with concerns about the likely impact on safety surely top of the list.
"Being able to save people from burning buildings or rescue them from the scene of a multiple vehicle pile-up requires not just bravery, but a good deal of physical strength too.
"There won't be many members of the public who would feel confident about being rescued from a serious fire if the only route out of the inferno was down a ladder on the back of a firefighter who was about to turn 60. Yet that is exactly what the FBU fears could happen if the Government gets its way.
"Ministers might say that they'd move older staff off frontline duties but the reality is likely to be very different.
"The Government's own figures show that thousands of firefighters in their 50s could face the sack without much of a pension, simply because they would no longer be able to meet the required fitness standard."
Fire brigades have drawn up their own contingency plans to deal with the strike because military Green Goddess machines, which have been used as cover in previous disputes, are no longer available.
They were sold or donated several years ago to countries including Zambia, Paraguay, Uruguay, Bosnia, Peru, Bolivia, Albania, Kosovo, Montenegro, Argentina, Pakistan, Sri Lanka and Sierra Leone. They have been sold by the Ministry of Defence to countries including Malta.
The public has been told that 999 calls will be answered, but brigades added that some non-emergency calls such as those to floods or rubbish fires will not be dealt with.
Almost 80% of FBU members voted in favour of industrial action in a ballot that ended earlier this month, although union officials said they left the strike to the last possible moment to allow for the possibility of a negotiated settlement.
Firefighters in Scotland will not be joining the strike while union officials discuss proposals put forward by the Scottish Government.
A spokesman for the Communities and Local Government Department said: "All fire and rescue authorities now have a statutory duty to put continuity plans in place for emergencies, including strike action, under the Civil Contingencies Act 2004.
"Many are using experienced officers and retained or on-call firefighters who are not part of the FBU action.
"The Government will be closely monitoring the situation throughout and has a strategic back-up it can deploy if needed."
A Ministry of Defence (MoD) spokesman said: "We can confirm that the MoD is providing support to the Department for Communities and Local Government (DCLG) and the Welsh Government during industrial action by the Fire Brigades Union.
"In England the MoD is providing a strategic reserve to be deployed only in the event of a major incident, while in South Wales MoD personnel are fulfilling the role of frontline firefighters. The MoD is also providing a reserve in South Wales should more capability be required.
"The personnel being deployed are professional Royal Navy and Royal Air Force firefighters with fully trained Army crewmembers in support and they are using civilian appliances."