The evening rush hour in London started early as workers tried to get home before a strike closed the capital's entire Tube system.
Members of four unions are taking industrial action for the second time in a month because of a deadlocked dispute over plans to launch a new all-night service next month.
No underground trains will run this evening or tomorrow, with the service returning to normal on Friday.
Extra buses will be laid on, but commuters and tourists face huge disruption and London's roads could be gridlocked if people switch to cars.
London's mayor, Boris Johnson, made it clear no more money will be offered to resolve the row over pay and conditions, urging unions to put the latest "incredibly generous" offer to their members.
Speaking after a visit to military veterans at the Royal Hospital he repeated his refusal to meet union leaders, and said he was "not fussed" about the new service starting on time on September 12.
"I want it starting in the autumn - what I am fussed about is the offer being put to union members.
"I am not going to authorise any more money. Most people would recognise that this is a very generous deal."
Mr Johnson said he did not believe politicians should "undermine" their own management.
He added it would be "grotesque" if unions called longer strikes if the dispute remains unresolved.
The row escalated today when the RMT union announced it will now ballot engineers working for Tube Lines for strikes over the same dispute, and Aslef accused LU of "forcing through" new staff rosters for the night Tube without consultation
Aslef general secretary Mick Whelan said: "Our members have rejected the latest offer from the company because they are forcing through new rosters without agreement and offer no firm commitments on work life balance for train drivers.
"We support night Tube. London needs it. We remain prepared to talk at any time to try to find common ground but by forcing these new rosters on train drivers and acting outside of our agreed way of working senior management are making it even harder to resolve this dispute.
"The Aslef executive committee meets next week and will discuss our response to these developments."
RMT leader Mick Cash said: "The offer tabled by London Underground is just a rehash of an earlier package and does nothing to tackle the fundamental issue of our members being called into work at the beck and call of management to plug staffing gaps in the Mayor's botched night Tube plans.
"This dispute is not about money, it's about being able to plan for and enjoy some downtime with friends and family away from work. The current plans wreck that and the unilateral issuing of the new rosters has simply inflamed the situation."
He said the RMT had consulted its network of elected representatives who had universally rejected the latest offer."
Steve Griffiths, LU's chief operating officer, said: "We have made every effort to reach agreement with the unions and avoid this unnecessary strike action. On the table is an extremely fair offer.
"We have employed 137 new drivers and 345 new station staff for the night Tube service. We've made work-life balance guarantees that no-one will work extra hours and that drivers will have the same number of weekends off as now and will be able to choose whether they work night Tube shifts in future.
"Our staff and contractors on our other transport services will step-up once again to support London's workers, residents and visitors get around during the strike. I thank customers for their patience as they make their journeys this evening and tomorrow."