Emergency crews who worked during an eight-hour firefighters' strike faced "intimidation and harassment", London Fire Commissioner Ron Dobson has said.
Footage has emerged showing a group of people surrounding a fire engine returning to the fire station at Southwark Bridge Road, south London.
Images and names of some of the contract workers were put on a Facebook page set up in support of the strike.
The Fire Brigades Union denied engines were stopped en route to incidents.
More than 5,000 firefighters walked out on 23 October in a row over new contracts, including changes to shift times.
London Fire Brigade (LFB) said it had deployed 27 fire engines which were staffed by 162 contract staff across the city during the strike. The crews attended 49 incidents during the strike period.
Footage sent to BBC London and a video on YouTube show protesters shouting "scab" and a fire engine receiving a police escort as it returned to a fire station on Southwark Bridge Road.
Mr Dobson said he was at the fire station when it happened.
He said: "What I saw from this window on Saturday evening was the worst thing I have ever seen in 31 years, which was a fire appliance on its way up Southwark Bridge Road and a group of people in fire tunics and demonstrators running after it, intent on obviously stopping it and getting on it.
"Its fair to say emergency fire crews were intimidated and stopped from attending incidents on Saturday."
LFB will review its security plans ahead of the next eight-hour strike planned to take place on 1 November, Mr Dobson said.
Matt Wrack, general secretary of the Fire Brigades Union, said the "footage wasn't taken during the strike action" and it was therefore misleading to claim that fire engines were stopped "from attending emergency calls".
"That fire engine was returning to the depot at the end of the strike. Our members (were), quite rightly, justifiably angry.
"We liaised with the police all day long, we'd made clear our views to the fire brigade to speak to the private contractors who were being brought in to break the strike.
"They (contract workers) were poorly trained, they had been told they will be working alongside London firefighters, so there's a bit of misleading information being provided."
Earlier the Metropolitan Police told the BBC they did not receive reports of any serious incidents during the strike.