The Rail, Maritime and Transport union (RMT) and the Transport Salaried Staffs Association have begun a series of strikes on the London Underground following the failure of talks over job cuts.
Further strikes are planned for October and November.
The strike has disrupted the capital's rail operations, which carry about three million passengers a day.
The strike follows London Underground's refusal to withdraw plans to cut 800 station jobs and close ticket offices.
The union claims the plan will threaten passenger safety.
London Underground has, however, promised there will be no compulsory redundancies and said the talks failed because of the pre-conditions laid down by the unions.
The London Mayor and Transport for London have organised extra buses, escorted bike rides and marshalled taxi ranks to help people during the strike.