Guards on Southern trains walked out on strike for the 29th time in their long-running row over safety.
Talks between the Rail Maritime and Transport Workers union (RMT) and Southern broke down last week in the 10-month-long dispute.
It centres on Southern's plan to make conductors on-board supervisors, giving drivers control of carriage doors.
Southern said it expected to run "about three-quarters of its normal service", despite the action.
Angie Doll, passenger services director at Southern, advised passengers to expect some disruption.
The RMT union said the strike was "solidly" supported.
Mick Cash, general secretary, said members were "standing firm in their action in defence of rail safety".
"Our members' resilience for nearly a year now is a credit to the entire trade union movement.
"These are local people fighting for safe railways for their local communities."
'Lines of communication open'
The 24-hour strike, which began at 00:01 GMT, came after ongoing talks ended on 14 February without a deal.
Two days later, members of the train drivers' union Aslef voted against a deal which union leaders had agreed with Southern.
The union held talks with the rail firm on Tuesday, but there are no plans for any further discussions for the rest of this week.
An Aslef spokesman said it was reopening negotiations at the earliest opportunity since the ballot result, but given the length of previous debates a quick resolution was not expected.
Southern's parent firm Govia Thameslink (GTR) added: "All we can say is that lines of communication are open."