Workers on London's Underground train system started a 24-hour strike over staffing cuts on Monday, threatening disruption for millions of commuters.
Thousands of workers walked out during the evening rush hour at 5:00 pm (1600 GMT), with another wave due to stop work at 9:00 pm (2000 GMT) in protest at plans to axe 800 jobs in a cost-cutting measure.
The mayor of London, Boris Johnson, criticised the industrial action - which will be followed by further strikes in October and November - as a "trumped-up and politically motivated" attack on the coalition government.
But the Rail Maritime and Transport (RMT) union highlighted three recent incidents - fire scares at Euston and Oxford Circus stations and the arrest of a man with a sword and two loaded guns - as reasons for maintaining staffing numbers.
RMT general secretary Bob Crow 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, and yet he is failing to take any account of the hard facts of these three recent incidents - each of which could have had lethal consequences."
Unions claim that although the majority of the jobs to be cut are in ticket offices, the staff contribute to the general security of Tube stations.
London transport chiefs have laid on 100 extra buses in an attempt to keep the British capital moving.