More than 100 protesting firefighters greeted Mayor Boris Johnson and London Fire Brigade officials at the official opening of a new fire station.
They were protesting against new shift patterns, which will see them working shorter nights and longer days.
London Fire Authority said the changes were a part of a modernisation plan but firefighters said the proposed 12-hour shift pattern could endanger lives.
The protest took place at Harold Hill fire station in Ashton Road, Romford.
The mayor visited the facility, the first to be opened in the new millennium, with Councillor Brian Coleman - chairman of the London Fire and Emergency Planning Authority, and London's Fire Commissioner Ron Dobson.
On the protest Mr Coleman said: "I'm entirely relaxed that we are pushing forward the modernisation agenda, that we are investing in the London Fire Service and that we are making London a safer place for all Londoners, including union members."
'Not an option'
But Ian Leahair, from the Fire Brigade Union, said: "We know for a fact that 12-hour shift proposals will lead to a worse fire service in London.
"The current shift system has been operating for nearly 30 years and the old saying's there - if it isn't broken don't fix it."
Mr Dobson said he was "very keen" to negotiate a settlement with the union adding: "One thing is clear that no change is not an option."
The mayor avoided commenting on the demonstration, saying the firefighters were present to "join in" the opening of the station.
The new station has state-of-the-art indoor training facilities and comes fitted with recliner chairs, instead of beds, for firefighters to rest in during the night shift.
The station also has technology to recycle rainwater, solar panels, energy saving boilers and motion sensor lighting.
On the station, Mr Johnson said: "Harold Hill is a superb example of how you can combine both value for money and green ambitions."