LONDON -- Many London commuters left work early Wednesday to beat the start of a strike that's expected to shut down the city's subway system.

Drivers and station staff were walking out for 24 hours from 6:30 p.m. (1730GMT) in a dispute over pay and schedules when a 24-hour subway service starts on some lines later this year.

Subway operator Transport for London says so many drivers are expected to participate that it will be impossible to run any trains during the walkout. In past strikes, management managed to keep a few lines running.

The Underground handles 4 million journeys a day, and the strike by members of four unions will likely paralyze the capital's transport system, despite extra bus and river services.

Union officials accused management of failing to negotiate seriously, but London Mayor Boris Johnson branded the strike "totally unnecessary, cynically timed and politically motivated."

Prime Minister David Cameron's spokeswoman, Helen Bower, described the strike as "unacceptable and unjustified."

"It's going to hit families, workers and businesses across the capital," she said.