The Rail, Maritime and Trade Union (RMT) has begun its six planned dates of industrial action on Tuesday (April 26) with a protest at Victoria Station.
The strike is being held because union bosses say Southern Railway plans to cut guards on trains throughout Surrey and Sussex. The RMT is against plans for driver only operated trains, which it claims are less safe for passengers.
They do not have a guard which means the driver, aided by CCTV, is the only staff member responsible for passenger safety.
Unions have been using a dramatic image of a woman dragged 30ft along the platform by a train after getting her hands stuck in the carriage doors as an example of why Southern conductors are striking.
RMT general secretary, Mick Cash, said at Victoria Station today: "Southern trains have tried to get rid of the safety critical guard on the train. We've got two people who make sure the train is safe, the driver who sits at the front of the train, and the guard who is with the passengers.
"We think that's unsafe, and therefore we are taking industrial action, to make sure we can maintain the safety of our railways and the passengers.
"We want Southern to withdraw their proposals to get rid of the second safety critical worker protecting the train, we want them to sit down with us and see how they can run a safe and efficient railway without getting rid of safety critical workers making the railway unsafe."
"We don't want profit being put before safety."
A spokesperson for the RMT said: "Currently, both the driver and the guard protect the safety of the train.
"Guards must be fully trained in operational safety and route knowledge, including being able to secure the doors safely, protecting the train and acting in emergencies such as derailments or fires.
"A passenger was dragged along the platform after her bag was trapped in the door of a driver only operated train at Hayes and Harlington on 25 July 2015."
The Rail Accident Investigation Branch is investigating the incident.
The first planned strike will be taking place on Tuesday and Wednesday (April 27), when Southern Railway conductors walkout with no service on many routes. No west London stations are likely to be affected by the action.
More information on the strike and how it will affect your journey can be found on the Southern Railway website .