National Union of Rail, Maritime and Transport Workers General Secretary Bob Crow said support was "rock solid" for the strike, which disrupted commuters Wednesday. Plans call for the strike to go on through Thursday and then to be repeated Wednesday and Thursday next week, the Guardian reported.
Crow's union and the Transport Salaried Staffs' Association called the strike to protest a plan to eliminate 953 jobs, including those of ticket seller. The unions say the layoffs would pose safety concerns.
London Mayor Boris Johnson made a pledge in 2008 to keep ticket offices staffed but now says technology has eliminated the need for the positions.
The "pointless strike which will do nothing other than cost your members their wages," Johnson said in a comment directed at union members.
Prime Minister David Cameron called the strike "shameful, bringing misery to millions of Londoners."
Transport for London, the government agency that oversees public transportation in the city, said 82 percent of respondents indicated support for closing the system's ticket offices, but the RMT union said the results of its survey found a majority supported the strike.