Thousands of drivers, station staff and signallers on London Underground are staging the walkout due to safety concerns over the government's public-private partnership (PPP) plans.
Bob Crow, the general secretary of the rail, maritime and transport union, said he expected solid support from his members and claimed that LU had treated the union's safety fears with "contempt".
LU warned the 3 million passengers who use the Tube every day that services will be "severely disrupted". Anyone using the underground, especially those heading for airports, was urged to complete their journey before the 8pm deadline.
Services will be crippled tomorrow and will not return to normal until Friday morning.
LU said that "very few" trains would run during the strike, which will cause massive disruption for workers and visitors in the capital. The strike will cost the economy £60m, Maurice Fitzpatrick, head of economics at accountants Tenon, said today, although other estimates have put the figure closer to £100m.
Buses and trains will be even more packed tomorrow as people find other ways of getting to work, although some office staff were expected to work from home.
Transport for London has produced "walking maps" to encourage people to walk rather than queue for buses. A free boat service will operate during the morning and evening rush hours along the River Thames in central London, capable of transporting 1,500 people an hour.
London's transport commissioner, Bob Kiley, voiced doubts about the Tube strike.
He said: "While it's no secret that I have serious concerns about Tube safety under the proposed PPP, strike action is unlikely to help keep the support of London's travelling public. I urge the parties to return to the negotiating table and the government to drop their discredited Tube PPP plans."
The union has not called any more strikes, but further industrial action was not being ruled out.
