The union is striking against changes it claims will mean firefighters will have to work longer, pay more into their pensions and receive less in retirement.
Ministers have said the action is unnecessary and the pension offer is one of the most generous in the public sector.
About 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.
The FBU claims many firefighters will not be fit enough to work to the retirement age of 60 and 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."
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.
Some brigades, such as London and Surrey, are hiring private contractors to stand in for striking firefighters, but the public has been warned that some non-emergency calls will not be answered.
Fire minister Brandon Lewis said as the FBU is going ahead with the strike public safety remains its 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.
"The Government has listened to union concerns. Firefighters will still get one of the most generous pension schemes in the public sector."
