Workers on London's Underground train system begin the latest in a string of 24-hour strikes over staffing cuts on Tuesday, threatening fresh disruption for millions of customers.
Following earlier strikes in September and October, thousands of staff are set to walk out during the evening rush hour from around 1900 GMT in protest at plans to axe 800 jobs.
Mayor of London Boris Johnson said people in the capital "will not be deterred from their daily business by these pointless strikes" and pledged to lay on extra bus and boat services to help ease disruption.
Services are not set to return to normal until early Thursday. About 3.5 million Underground journeys are made on a normal weekday.
Union leaders - who plan another stoppage at the end of November - say the job cuts will affect safety and accused Johnson of dropping a pledge to keep Underground stations properly staffed.
Bob Crow, the leader of the Rail Maritime and Transport (RMT) union, said Tube workers had last week been commended at an inquest into the July 2005 bombings on the London transport system "yet among them are the very grades that the mayor is now intent on cutting."
Unions claim that although the majority of the jobs to be cut are in ticket offices, the staff contribute to the general security of Tube stations.
The planned job cuts come amid an austerity drive in Britain, where the coalition government last month announced average spending cuts of 19 per cent across departments.
BBC journalists and London firefighters are both set to stage strikes Friday and Saturday, while the firefighters also held an eight-hour walk-out on Monday.