Millions of passengers faced severe disruption as services on all but the Northern Line were suspended or delayed for most of the rush hour.
The unions said support for the action was “rock solid,” with each day expected to cost the capital’s economy £50million.
Maintenance staff walked out at 1700 BST on Monday and drivers, signallers and station staff stopped work four hours later in a row over job cuts, causing misery for millions of commuters and other passengers.
View the effect the strike is having on the underground using our live live and interactive widget:
A full service has been operating on the Northern Line but many stations on the line are closed.
There are no trains running on the Circle, Piccadilly, Victoria and Waterloo and City lines.
The Bakerloo, District, Hammersmith and City, Jubilee and Metropolitan lines are partly suspended and there are severe delays on the Central Line.
An extra 100 buses and 10,000 more passenger journeys on Thames riverboat services have been laid on. Some taxi ranks will be marshalled and escorted bike rides will be operating.
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