Drivers on five London Tube lines have gone on strike on Friday - causing widespread problems.
Services on the Victoria, Central, Jubilee, Northern and Piccadilly lines are affected.
Transport for London (TfL) has warned people to expect "severe disruption".
The strike began at 4:30am on Friday, and runs for 24 hours.
TfL said: "Stations will be open as usual, however, some tube lines will operate a much reduced service or no trains at all."
Talks of the strike began because of disputes over pay and hours for drivers of the Night Tube, which was reopened after being suspended in March 2020 during the pandemic.
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Mick Lynch, general secretary of The Rail, Maritime and Transport union (RMT), 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."
Commuters took to Twitter on Friday to vent about the strike.
One person said: "Oh yay! Nothing like a tube strike to get Friday started."
Another added: "Really regretting no being able to work from home today.
"The weather is awful, Covid cases are rising (got a close contact email yesterday) and I had to leave home extra early due to the tube strike."
Planned walkouts are due to continue on different days until 18 December, unless a deal can be reached.
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Another strike will be held on Saturday evening on the Central and Victoria lines, hitting plans to restart the Night Tube this weekend.
RMT 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.
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."
