"This action has been forced on us by savage cuts to jobs that have reduced London Underground to an under-staffed death trap at a time of heightened security and safety alert," said Mick Cash, general secretary of the Rail, Maritime and Transport Union.
While acknowledging some issues needed to be addressed on the network, London Mayor Sadiq Khan described the strike as a "completely unnecessary" action that was causing inconvenience across the city.
"Why strike?" Khan told the BBC. "Why not resolve these things amicably?" London's subway system handles more than 4 million journeys during a typical business day.