The 48 hour walkout started at 21:00GMT on Tuesday.
The RMT and TSSA unions want Transport for London to suspend plans to close all ticket offices, and therefore 950 jobs would have to be cut. However the authority says the plans are needed to save £50 million a year.
Picket lines have been set up at Underground stations across the city, many of which will be closed.
About 100 extra buses will be laid on and normal services will run on London Overground and Tramlink, but the strike is expected to cause major travel disruption to millions of passengers.
On Tuesday, London Mayor Boris Johnson described the action as “pointless” and said he wanted to change strike laws, introducing a minimum “threshold” of support for industrial action before it could go ahead.