London Underground services were disrupted on Friday after drivers launched a 24-hour strike in a dispute over the Night Tube.
The Rail, Maritime and Transport union (RMT) said its members were solidly supporting the walkout, which will be followed by more action in the run up to Christmas.
Picket lines were mounted outside some Tube stations from early morning.
The strike affected the Central, Jubilee, Northern, Piccadilly and Victoria lines.
Another strike will be held on Saturday evening on the Central and Victoria lines, hitting plans to restart the Night Tube this weekend after it was suspended last year because of the pandemic.
The union says Transport for London (TfL) has “ripped up” an agreement on Night Tube driving by changing rosters.
TfL said no jobs are being lost and the changes mean drivers would work around four Night Tube weekends a year.
RMT general secretary Mick Lynch said: “The Tube strike action this morning is rock solid in all depots and the widespread impact on services is solely down to management failure to recognise and address the anger of their staff at the imposition of damaging and unacceptable working practices.
“This action was wholly avoidable if LU bosses hadn’t attempted to bulldoze through arrangements that abolished the Night Tube driver grade, lumping everyone into a central pool where they can be shunted about at will in a drive to cut costs.
“Our members have spoken this morning and it’s time for London Underground to start listening.
“This is just the start of a programme of action and the Mayor and his officials need to recognise our determination to defend progressive and family friendly working practices. We remain available for talks.”
Nick Dent, director of London Underground customer operations, said: “At such a pivotal time for the capital’s recovery, we are hugely disappointed that the RMT is threatening London with this unnecessary action.
“By making changes to Tube driver rosters we have provided greater flexibility for drivers as well as permanent work and job security, something welcomed by all other unions.”
Andy Lord, managing director of London Underground, said: “We are currently running 58% of our usual scheduled service, despite the RMT’s strike action on five London Underground lines today, but there are significant impacts on the affected lines.
“This frustrating strike action is the last thing that London needs as it recovers from the pandemic and we have done everything we can to get this unnecessary action called off.
“We are doing all we can to provide as good a service as possible for Londoners and visitors on the Tube network today, with staff and travel ambassadors on hand at stations to assist customers. We’re advising everyone to check before they travel and leave more time for journeys.”