Police were called to London Bridge station this evening after thousands of travellers were left stranded when services ground to a halt during rush hour.
Passengers facing delays were told there were signalling problems south of the station.
A spokesman for Network Rail confirmed that the delays had been caused, in part, by a person being hit by a train.
The spokesman said: "There are currently three delays and alterations to southern services due to a person being struck by a train."
Commuters were angry amid the chaos, with many expressing their anger on Twitter.
Joe Kallas tweeted: "Avoid London Bridge like the plague. @SouthernRailUK you have outdone yourselves."
Meanwhile Gabriella Griffith tweeted: "London Bridge has reached peak chaos."
Network Rail said in a statement it was hoping to return to normal service as soon as possible.
The statement said: "There was an incident this afternoon where a person jumped onto the tracks at Streatham.
"We turned off the electricity which powers the trains to prevent any further harm to the individual and British Transport Police were called to manage the incident and remove the person safely from the railway.
"While the railway has been open since 1727 and this happened on a separate route to that serving London Bridge, it did cause a considerable amount of knock-on delays to other services, including those out of London Bridge tonight.
"We're sorry this incident has caused the disruption it has and we're working as hard as we can to recover the service and get as many people where they need to be tonight."
British Transport Police were called to the station earlier this evening to help manage the growing crowds.
A spokesman said: "We are still there helping with crowd control. I can't say at this stage how much longer we will be there for."
Services on Southern Rail were severely delayed this evening, with many customers tweeting their outrage after being left unable to get home.
One angry traveller even tried to get her fellow passengers to sign a petition asking Southern Rail to improve their service.
Sachika Taniyama tweeted: "Can we all sign this? I pay £200 per month to this company @SouthernRailUK and all it brings is misery to my day."
Ms Taniyama also tweeted a link to an epetition demanding that Southern Rail "be held accountable for its rising fares and diminishing customer service, which cause misery to thousands of passengers daily".
A spokesman for Southern Rail UK said: " Unfortunately, a series of incidents including a person being struck by a train have affected evening peak services from London.
"Delays of up to 60 minutes, cancellations and short notice alterations to services have been experienced by our passengers for which we apologise.
"Our main focus is on getting everyone home and we are working hard on that at the moment."