Striking ambulance staff have been forced to leave picket lines to attend emergency calls, union official have said.
Thousands of workers, including paramedics and call handlers, walked out across the country on Wednesday morning in a row over pay and conditions.
Before the strikes began, union officials said they would continue responding to all Category 1 calls - the most life-threatening requests, such as cardiac arrest.
Nathan Holman, the GMB Union’s South West and Wales representative, said all its striking members had to be called away from the picket line in Cardiff to attend emergency calls.
Mr Holman tweeted a video of himself alone at Pentwyn Ambulance Service depot just before 8am, with the caption: "Unfortunately, all the members have had to go because, as you can see behind me, there’s only one vehicle left, they’ve all jumped on a vehicle and responded to emergency calls.
"So, just shows we’re still responding to the public."
Jenny Withall joined her colleagues on picket lines in Coventry for twp few hours on Wednesday morning before having to return to work when a 999 call came through.
Ms Withall, 32, a paramedic since 2016, told South West News Service: “I am striking to try and sort the NHS out and help people out there struggling that we are just not getting too because of the working conditions.”
Around 1,500 ambulance staff in Wales were expected to take part in Wednesday’s industrial action.
Crews are still required to respond to critical 999 calls, but despite this, the service is expected to be "significantly impacted", according to the Welsh government.
The strikes in Wales went ahead after unions and the Welsh government failed to reach a breakthrough on pay and conditions.
The Welsh government gave ambulance staff a pay rise of 4.75 per cent but unions are calling for hikes in line with inflation, currently running at 10.7 per cent.
Ministers said those demands were unaffordable without more funding from Westminster.
Ambulance workers in England are also walking out over pay and conditions.
Steve Barclay, the health secretary, said the government would stick to the recommendation of the independent pay review body which deals with salaries.
It gave workers a pay increase of 4 per cent but ambulance staff in England are also calling for inflation-proof pay rises. Ministers decide whether or not to accept recommendations made by pay review bodies.
Almost all ambulance trusts in England are at such a level of disruption that they might not be considered safe and patients might face harm, with health bosses on Tuesday night saying they were unable to guarantee patient safety.
The majority of ambulance trusts have declared so-called critical incidents, with many trusts stating that they were facing huge pressure before strikes began on Wednesday.
Ministers, meanwhile, have urged Britons to avoid risky activities and not drink too much alcohol at Christmas parties during the strike action.
