Millions of Londoners face travel misery after members of the Rail, Maritime and Transport Union walked out for 48 hours in protest against job cuts.
Transport for London said in a statement that it expected about 40 percent of services to run and that an extra 266 bus services were being made available.
Some trains were running on nine of 11 London Underground lines on Tuesday.
TFL suggested Londoners used river services or cycled and walked as other "practical options" for transport.
The tube workers have walked out over plans to close all ticket offices at a cost of nearly a thousand jobs.
- Strike criticized
The 48-hour strike, which kicked off at 9 p.m. (2100GMT) on Monday evening, is expected to end at 9 p.m. on Wednesday.
However, there are warnings that many services may still be affected on Thursday morning.
British Prime Minister David Cameron criticized the strike by sending a message from his official Twitter account saying: "It's unacceptable that millions of people are having their lives disrupted by today's Tube strike in London."
Even though the TFL has said it will continue talks with unions, it has insisted on the job cuts.
The TFL is seeking £50 million (US$84 million) worth of savings.
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