The 24-hour strike, which ends at 11am but will effect journeys throughout the day, has been blasted as "completely unnecessary" by Southern rail.
However the Rail, Maritime and Transport union argued that the strike, along with two more walkouts planned for next month, are essential to secure the safety of passengers, with proposals to reduce train staff levels and shift responsibility for opening doors from conductors to drivers.
The union said commuters are being treated like "self-loading freight" and said they had the support of the public - despite the ongoing inconvenience caused by the strike.
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Southern sees their passengers as nothing more than self-loading freight who can be ripped off at every turn
Southern said it ran around two-thirds of its services on Tuesday and that, while trains were full, crowd control measures were not necessary at stations. Some routes off the mainline were either suspended or running with a reduced service.
A spokesman for Southern said: "This strike by the RMT is completely unnecessary and causes undue misery to our passengers. "No staff would lose their jobs under our proposals or see a reduction in their salary, whilst passengers would benefit by having more visible staff on trains. We urge the RMT to come back to the negotiating table.
"As many trains will be staffed as today - we simply want to evolve the role of our on-board staff to make them more visible and available to passengers. In addition, services won't be cancelled if an on-board supervisor is not available.
"That in itself - particularly in times of disruption - is a massive bonus for our passengers.
"Journeys today will continue to be difficult and frustrating for our passengers. We hope to run as many trains as possible, but on some routes there will be no train service at all and on others it will be significantly reduced and start late and finish earlier than usual."
Man has pizza delivered to him on a train
The union disputed the number of services Southern said it ran and maintained it had public support.
General secretary Mick Cash said: "The public pay thousands of pounds a year to travel on Southern services and they expect safe staffing levels and decent services and rightly so.
"Southern sees their passengers as nothing more than self-loading freight who can be ripped off at every turn and it is no wonder that the public are backing their safety-critical guards in this dispute."