British PM Theresa May condemned the five-day rail strike which affected hundreds of passengers.
Railway workers started a five-day strike on Monday, disrupting services for hundreds of thousands of passengers using the UK's southern network, even as British Prime Minister Theresa May condemned the move.
Hundreds of trains have been cancelled in a row over plans for drivers, instead of conductors, to operate carriage doors on the trains.
"The Prime Minister strongly condemns the strike action It's only going to cause more disruption and misery for passengers. We are deeply disappointed that union bosses are overlooking the impact that they are having on the public, and we obviously want to see the situation resolved," a Downing Street spokesperson said.
The RMT union claimed "rock-solid support" for the strike and said the central issue was safety. The union has held a series of one-day strikes since April after balloting 393 members.
"This action has been forced on us by the arrogance and inaction of Govia Thameslink [parent firm for Southern railway] and the government who have made it clear that they have no interest in resolving this dispute or in tackling the daily chaos on Southern," RMT general secretary Mick Cash said.
"Our fight is with the company and the government who have dragged this franchise into total meltdown. We share the anger and frustration of passengers and we cannot sit back while jobs and safety are compromised on these dangerously overcrowded trains," he said.
Govia Thameslink chief executive Charles Horton said the RMT action was "completely unacceptable, unjustified and unnecessary" and he apologised for the "inconvenience and disruption to passengers' lives as a result".
Southern is the main operator for Sussex and east Surrey regions of England, with services running to London, Kent, Hampshire and Buckinghamshire.
Last month, it brought in a reduced timetable in response to continued cancellations and delays, blaming issues with crew availability.
The Campaign for Better Transport and a Southern passenger group, the Association of British Commuters, plan to march from London Victoria station to the Department for Transport on Wednesday afternoon to hand in a "six-foot letter".
It calls on the new rail minister, Paul Maynard, to "listen to passengers, freeze fares and pay compensation".
Britain's railway network is run by a series of firms like Southern since it was privatised in the 1990s.