The evening rush hour in London on Thursday was thrown into chaos by the network-wide closure of the Tube over an employment dispute.
The strike over pay and night services is causing travel misery for millions.
Separately, a strike by First Great Western staff disrupted travel between London, the west and Wales.
An extra 200 buses are being laid on, as well as additional river services, to help cope with demand in the capital, Transport for London (TfL) said.
Those able to travel outside peak times are being urged to do so.
TfL said Tube services would resume on Friday with a full service, but advised customers to check before they travel.
On Thursday morning Mike Brown, managing director of London Underground, said: "I am very sorry your journey has been disrupted. This strike is unnecessary."
The strike began at 6.30pm Wednesday when members of the Rail, Maritime and Transport (RMT) union, the Transport Salaried Staffs Association (TSSA) and Unite walked out in a 24 hour action.
later that night members of the train drivers' union Aslef began their own 24 hour stoppage.
RMT general secretary Mick Cash said: "The strike action on London Underground is rock solid across all lines.
"That means an end to the attempt to bulldoze through new working patterns."
London mayor Boris Johnson has refused to rule out a delay to the opening of night Tube services, which are at the centre of the dispute between unions and London Underground.
TfL warned rush hour was likely to start earlier on Thursday as people made alternative arrangements to get home. It said buses, Overground services, river and Emirates Air Line services, as well as the cycle hire scheme were all likely to be in high demand when people headed home.
On Monday, members of all four unions rejected a "final" pay offer from LU which included a 2 percent rise this year and £2,000 for drivers on the weekend night Tube service.
The typical salary for a Tube driver is £50,000 a year, the RMT said, but the unions maintain the new plans would be disruptive to their members' lives.
They claim some employees are concerned they will have to work more overnight shifts and may have to work on their own at some stations.
The disputed weekend night services are set to begin on 12 September on sections of the Central, Jubilee, Northern, Piccadilly and Victoria lines.
About 20,000 Tube workers are expected to walk out during the course of the strike.