Transport for London, which runs the subway, said that about 40% of underground trains were operating, along with the Docklands Light Railway, which serves the financial district in the east of the city.
This is the third strike since September. Workers are protesting against plans to eliminate 800 jobs by reducing station staffing and opening hours at some ticket offices.
The subway carries 3 million passengers a day. Business lobbies claim the strikes cost as much as 50 million pounds ($80.7 million) a day.