The strikes began after talks between RMT and SWR broke down last week, over whether human guards should operate the opening and closing of train doors.
As the strike got underway passengers complained of “dangerously overcrowded” carriages and “scrums” ensuing as commuters struggled to board trains.
One passenger, Rachel Lonergan, tweeted SWR to say: “The train I was on this morning was dangerously overcrowded with fare-paying passengers feeling very unwell.”
Another, IT manager Karl Lawson, who is waiting for a knee replacement operation, spoke of how he had to bring a foldout chair to aid his commute.
The 59-year-old from Basingstoke said: “It has been very difficult. I need a knee replacement and cannot stand for more than half an hour.
“I had to bring my own chair even though I paid for a seat. I used it on the platform and on the train.
“It is a scrum to get to the train. There is no such thing as politeness. We were told there would be queues but everyone was just fighting to get on the train.”
Other passengers were also asking on social media whether they would be eligible for refunds on their season tickets during the strike.
During previous walkouts, such as on the Northern Rail lines, passengers have been able to claim back a day’s worth of fares if they did not travel due to strike action.
SWR said that any season ticket holder wanting to make a claim should contact their customer services and those claims would also be “reviewed individually”.