London Underground workers started a fresh wave of 24-hour strikes, threatening travel chaos for days and costing the economy almost £50 million.
Thousands of members of the Rail Maritime and Transport union and the Transport Salaried Staffs Association walked out at 5pm on Monday, with another wave at 9pm, in protest at plans to axe 800 jobs.
The action, to be followed by further stoppages in October and November, will disrupt Tube services, used by millions of passengers every day.
Mayor Boris Johnson criticised the industrial action as a "trumped-up and politically motivated" attempt to attack the coalition Government.
The strikes followed a 24-hour stoppage from 7pm on Sunday night by up to 200 maintenance staff at depots on the Jubilee and Northern lines in a separate row over pay and conditions, which the RMT said was "solidly" supported.
The RMT highlighted three recent incidents - fire scares at Euston and Oxford Circus and the arrest of a man with a sword and two loaded guns - as reasons for maintaining staffing numbers.
General secretary Bob Crow, who will join a picket line at Euston on Tuesday morning, said: "We have laid out the clearest possible evidence to the mayor and his officials that if he breaks his promises and slashes station staffing numbers he will be giving the green light to disaster."
Mr Johnson said new staffing proposals were "moderate and sensible" and accused the unions of "cynically deciding to try the patience" of commuters.