The trade union chiefs behind the London Underground station staff strike have defended the 24-hour walkout, as the UK capital faces travel chaos on Monday (9 January).
RMT general secretary Mick Cash blamed "savage cuts" for the industrial action and said the solution was in the hands of Labour Mayor of London Sadiq Khan.
"This action has been forced on us by savage cuts to jobs that have reduced London Underground to an under-staffed death trap at a time of heightened security and safety alert," Cash said.
"RMT members will not stand idly by while they see day in and day out the safety regime on the Tube being slashed to ribbons.
"Even senior Tube bosses have admitted that we are absolutely correct in our assessment of the risks that are being taken as the impact of the 900 station job cuts hits home.
"The solution is in the hands of the mayor and his officials. They need to come up with serious and urgent plans designed to address the core issues at the heart of this dispute and a schedule for staffing back up on the stations to a level our reps believe is safe and sustainable. RMT remains available for further talks around that programme of action."
Manuel Cortes, the general secretary of the TSSA union, added: "Put quite simply these level of cuts are not compatible with a safely run, properly staffed Tube and my members are now highly anxious about the impact this is having and will continue to have on their ability to keep you safe.
"Our overtime ban of the last month has demonstrated effectively that there are no longer enough Tube staff employed to keep the network's 270 Tube stations open."
Talks between Transport for London, the RMT and its sister union, the TSSA, at mediation service Acas collapsed on Sunday. The strike is part of long-running dispute over former Mayor of London Boris Johnson's Fit for Future Stations programme.
The industrial action, which started at 6pm on Sunday, means the majority of stations in Zone 1 of the network have been closed.
TfL has promised to deploy 100 extra buses and enhance river services in a bid to avert travel chaos in the UK capital.
Khan has branded the strike as "totally unnecessary" and urged the unions to get back around the negotiating table.
"This historic dispute, which started under Boris Johnson, has nothing to do with the millions of Londoners this strike is punishing. It must be called off," he said. TfL remains open for further talks.
Steve Griffiths, chief operating officer for London Underground, said: "I thank customers for their patience as they try to make their journeys today during this unnecessary strike. We have hundreds of Travel Ambassadors on hand to help keep customers informed of what services are running and to help them get around the capital.
"This strike, called by the leadership of the RMT and TSSA unions is unnecessary. We had always intended to review staffing levels and have had constructive discussions with the unions.
"We agree that we need more staff in our stations and have already started to recruit 200 extra staff and this is likely to increase further as we work through the other areas that need to be addressed. Taking into account existing vacancies and natural turnover this means that over 600 staff will be recruited for stations this year. There will also be increased opportunities for promotion."