Tube workers have carried out a six-hour strike in protest at the sacking of a driver.
The Rail, Maritime and Transport union (RMT) alleges Arwyn Thomas was sacked for his union activities and demands his reinstatement.
London Underground (LU) said it will abide by a tribunal ruling due shortly and the strike would "achieve nothing".
It claimed the strike, which started at 2100 BST on Sunday, would cause little disruption.
It said a near normal service was expected on Sunday night and Monday morning and advised passengers to make their journeys as usual.
Three further strikes are planned.
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 were mounted at some Tube stations on Sunday night, and a rally in support of Mr Thomas was held at Golders Green.
At an interim hearing a judge suggested a tribunal was "likely" to find Mr Thomas was dismissed for his union activities.
Further planned strikes - including a 24-hour walkout - could affect people travelling to the Wimbledon tennis championships.
An RMT spokesman said: "We are prepared to enter into meaningful negotiations with London Underground to resolve this dispute."