Firefighters in England and Wales are due to strike for four hours from midday in a dispute over pensions.
Unions say it is a "warning shot to the government" over members' anger about making them work until they are 60.
Ministers say the action is unnecessary and the pension offer is one of the most generous in the public sector.
Each of the 46 local fire services in England and three in Wales have contingency plans to provide cover and 999 calls will be answered.
Firefighters in Scotland will not be joining the strike but could take action at a later date.
The contingency arrangements vary: in some areas non-unionised fire crews will step in and greater use will be made of part-time and volunteer firefighters.
Private contractors will be used by a handful of services, including in London and Surrey.
The armed forces have a back-up role and in the event of a major incident union members can return to work.
Analysis
Union officials see this strike as their opening shot in a dispute that could intensify. They are certainly not ruling out further industrial action in the weeks ahead.
As ever, public opinion will play a crucial role. Firefighters have traditionally enjoyed widespread public support. But the government is highlighting what it regards as the very "generous" terms of this pension offer.
Ministers say the arguments over fitness are a distraction - as the normal retirement age of 60 has actually been in place since 2006. But the FBU says those unable to do the job in their late 50s will be left without a "proper pension".
The strike will test the arrangements brought in since the last national dispute 10 years ago. Then the Army provided cover in 1950s "Green Goddesses".
But the Goddesses have all been sold and it is now up to each local authority to find part-timers, volunteers or private contractors to keep a basic service going.
BBC local government correspondent Mike Sergeant said many full-time firefighters were likely to strike.
The Fire Brigades Union (FBU) claims many firefighters will not be fit enough to work to the retirement age of 60, and says they could lose thousands of pounds a year if they retire early.
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."
The government says the offer to firefighters is one of the "most generous in the public sector" and brings their pension age into line with the police and armed forces.
Fire minister Brandon Lewis said: "Now that the Fire Brigades Union is pressing ahead with this unnecessary and avoidable strike, maintaining public safety is our first priority.
"I can reassure the public that all fire and rescue authorities in England and Wales have robust contingency plans in place.
"Incidents and call-outs are at an historical low - but in the event of a fire, the advice to people remains the same: 'Get out, stay out and call 999'; you will get an emergency response."
A spokesman for the Department for Communities and Local Government: "The government will be closely monitoring the situation throughout and has a strategic back-up it can deploy if needed."
Source: BBC News - Business http://www.bbc.co.uk/news/uk-24235251#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
