Southern strike: Tuesday 26 April
Southern rail had urged the RMT to return to the negotiating table before members walked out in a dispute over planned changes to the conductor role.
The 24-hour strike is the first of three days of industrial action planned for April and May.
Tom Edwards
Transport Correspondent, BBC London
Southern trains say they are planning to run 60% of services today.
Hundreds of RMT members walked out at 11:00 BST on Tuesday.
Image source, RMTThe union took this picture of striking conductors at London Victoria earlier.
The RMT says it takes no pleasure in members having to walk out but the dispute is about "safety".
BBC South East reporter Ali Fortescue is at Three Bridges railway station.
There are no Southern train services in either direction on the following routes:
The RMT says the strike has begun and there is "rock solid" support from all its members.
A spokesman told the BBC picket lines had gone up and he claimed there was strong backing from passengers.
Southern rail conductors are expected to walk out in the next few minutes in a dispute over planned changes to their role.
Here are some facts and figures about the train operator.
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Southern rail say "many trains" have been busy with "full and standing" passengers so far this morning.
A spokesman for the operator says at the moment they "feel confident" they will be able to run the planned revised timetable of 1,400 trains - 700 fewer than on a normal day.
They are waiting to see how many of their 470 conductors are available from 11:00 BST.
Dyan Crowther, chief operating officer at Govia Thameslink Railway, tells BBC Sussex that driver-only trains are a "safe method" of working.
"Over 60% of our services already operate from a driver-only perspective. Our rail regulator, our safety regulator also says it is a safe method of operation," she adds.
Image source, Southern rail"[The rail industry now has] better use of technology, our new rolling stock has in-cab CCTV that the drivers can use.
"There are an awful lot of things that have evolved over the years so we can evolve the role of our conductors."
Reporter Ali Fortescue is reporting from Three Bridges railway station.
She says trains are "on time at Three Bridges - but reports of delays further up the line in Oxted".
David Winter emailed to say Southern were "abolishing guards' jobs" and not changing conductors' roles, despite the operator's claims that this is not the case.
He writes: "They will only be allowed to sell full price tickets. They will have powers to penalty fare people without a ticket. At a time when ticket offices are being closed.
"Have you ever tried to get a discounted fare ticket from a ticket machine?"
Transport secretary Patrick McLoughlin says the conductor strike will inflict "unnecessary disruption" on passengers.
"I condemn any industrial action that will disrupt the travelling public. I expect to see operators and the unions working together to resolve these issues," he adds.
"It is clear that the changes GTR [Govia Thameslink Railway] are proposing will modernise services and provide better journeys for passengers."
Mick Cash, RMT general secretary, says the dispute is about "safety and the safety-critical role of the guards" on Southern trains.
"That is a lethal gamble with safety in the name of profit and that is why we have been forced to take strike action."
Operator Govia Thameslink says there will be no job losses or cuts in salary and the changes will make staff more visible.
Passengers have been telling BBC South East how the Southern rail dispute has been affecting their journeys.
Southern strike vox pops
You've been having your say on social media this morning.
Many people are talking about both the rail and junior doctors' strike.
Are you travelling in the South East this morning? Have you had to get an earlier train or change your plans because of the impending Southern rail conductor strike?
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The Conservative MP for Wealden, Nus Ghani, tells BBC Sussex that driver-only-operated trains are "happening on a third of the network already".
"I do not remember the last time I got on a train and there was someone to help me physically, on or off. Drivers can manage the doors," she adds.
Southern rail have created a map, external showing the level of service they expect to be able to offer passengers during the strike.
Image source, Southern railIt is colour-coded to show how badly affected they think lines will be. Red means there will be no service, yellow denotes a "significantly reduced service" and blue means reduced services.