More than 2,000 firefighters have attended a rally in Westminster to protest about changes to their pension scheme.
It was organised by the Fire Brigades Union (FBU) to coincide with strike action across England.
The FBU says changes to pensions and the retirement age are "unworkable".
A Department for Communities and Local Government spokesman said the strike was "unnecessary" and contingency plans were in place to deal with emergencies.
The FBU has accused fire minister Penny Mordaunt of misleading Parliament.
In a recent debate she told MPs that if a firefighter loses fitness through no fault of their own they should get another role or a full, unreduced pension.
FBU general secretary Matt Wrack told the BBC that firefighters signed up to a pension deal that is being ripped up.
He said: "They are not getting the pension they were promised. The pension changes take no account of the real job that they do, that's physically demanding.
"And they are paying huge amounts for it, over £4,000 a year on a salary of less than £29,000 a year.
That's one of the most expensive schemes in the public or private sector for a scheme that doesn't even promise they'll get paid anything in the end."
Around 2,000 firefighters crammed into the Methodist Central Hall at Westminster, while others remained outside
The 24-hour strike across England began at 07:00 GMT
A spokesman for the Department for Communities and Local Government said all fire and rescue authorities have contingency plans in place to cope with the walkout.
He said: "Public safety is our priority and if anyone needs emergency assistance they should dial 999 and they will get a response from their fire and rescue service.
"Strike action is unnecessary and appears to be over a point which is a vast improvement on the 2006 scheme which required firefighters to work to 60 with no protection.
"We have been clear that firefighters get an unreduced pension or a job and have changed the National Framework through a statutory instrument to do so."
The FBU says firefighters have been on strike over 50 times since September 2013. The last strike action was in December.