The union has been fighting the further extension of driver-only operation on the Southern network by Govia Thameslink Railway since April 2016.
The changes make drivers responsible for opening and closing train doors, with the RMT raising safety and access concerns.
As well as today’s strike, the union is also due to hold another 24-hour walkout on Monday September 4.
The RMT is also staging industrial action in two separate disputes with Merseyrail and Northern.
Mick Cash, general secretary of the RMT, said the support from its members was ‘solid as a rock this morning’, adding: “The RMT members on strike today across the country are a credit to their local communities and the entire trade union movement and it’s no surprise that the public have decided they are on the side of those fighting for safe, accessible rail services.”
Southern said it expects to run a normal service on most routes.
However trains on the Coastway route and between Hastings and Ashford International are expected to be affected.
Earlier Angie Doll, Southern’s passenger services director, said: “We plan to run more than 90 per cent of our services during the RMT conductors strike on Friday and Monday but ask passengers to check before they travel.
“Passenger numbers have doubled in the past two decades giving us the most congested network in the country so we simply have to modernise the way we do things to keep people moving.
“We ask the RMT to reconsider the very good offers we have made, including job guarantees, and work with us to future-proof services for our passengers - most of whom simply want to get to their place of work each day.”
Southern and Gatwick Express run more than 2,200 services on a non-strike day and most services will operate to the normal timetable.
Full details of service changes can be found on Southern’s website.
Passengers can claim compensation if they are delayed by 15 minutes or more here.