London Underground has maintained that a six-hour strike by Tube drivers will have "very little impact" on services.
Members of the Rail, Maritime and Transport union walked out at 9pm on Sunday for six hours in protest against the sacking of a driver, who is claiming unfair dismissal.
LU said a near normal service is expected throughout the strike, and it advised passengers to make their journeys as usual.
The RMT is calling for the reinstatement of the driver, Arwyn Thomas, who is waiting to hear from the tribunal whether he has won his case.
LU managing director Mike Brown said: "This strike action will achieve nothing and I urge the RMT leadership to calmly await the outcome of the employment tribunal without any further strikes.
"I have given a cast-iron assurance that if the tribunal rules that we should re-instate or re-engage Arwyn Thomas, then we will do so.
"We have given that assurance in writing before this strike action and I make it again now, so there can be no justification whatsoever for the RMT to continue to threaten Londoners with strike action."
Picket lines have been mounted at some Tube stations, and a rally in support of Mr Thomas has been held at Golders Green.
An RMT spokesman said: "We are prepared to enter into meaningful negotiations with London Underground to resolve this dispute."
The RMT is planning further strikes from June 27, including a 24-hour walkout, which coincides with the second week of the Wimbledon tennis championships.