Firefighters in London have walked out for eight hours in a row over new contracts that include changes to the shift times.
Thousands of members of the Fire Brigades Union (FBU) began their protest at 1000 BST.
The union claims London Fire Brigade (LFB) has threatened to sack those who do not sign up to the new shifts.
LFB has said it will respond to 999 calls, but said it may not be able to attend less urgent incidents.
London Fire Brigade said changes were needed so that the public would remain protected while budgets were squeezed.
At the start of the strike FBU general secretary Matt Wrack, who joined the picket line at Euston fire station, said he expected “solid support”.
He added: “London’s firefighters feel it is the only step they can now take.”
The union plans to walk out for eight hours again on 1 November.
A LFB spokesman said: “The Brigade’s contingency plan has been rolled out on schedule.
“All 162 contract staff providing the fire and rescue service in the capital today were deployed by 9am.
“During the day, 27 fire engines will provide the service, and throughout the day we will be reviewing our resources based on staff not taking part in the strike action,” he added.
London Fire commissioner Ron Dobson said: “We’ll be responding to 999 calls and doing everything we can to protect the public during this unnecessary FBU strike.”
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