Commuters arriving tonight from London Waterloo Station find themselves facing "total pandemonium" for about 40 minutes after an intruder on the tracks.
South Western Railway said in a statement that Wednesday saw an ongoing intrusion on the Surbiton-Earlsfield rail link.
The incident has now been resolved, but the closure of the main line has led to a long queue in which passengers were warned of disruption until 9pm.
Services may be delayed, canceled or changed after the disruption, she added.
Images shared on social media showed that many people gathered to enter the crowded London station while frustrated commuters complained of the "chaos" of the rush hour.
The Victoria Station was also closed because of a "fire alarm" at 19 clock.
One person wrote on Twitter: "At Waterloo there is complete pandemonium."
While another tweeted, "people are thrown out of the trains because the goals change after the board is displayed."
The passengers reported that "most of the trains" were canceled during the break.
In the meantime, long queues appeared in the pictures, winding around the station as commuters queued to get home.
In addition to a photo of the crowd, Ben Smith tweeted: "News about the normalization of trains in Waterloo, which ignore somewhat optimistic how difficult it is to enter the station."
It appeared that the entrances were closed, as reported on the social media.
In a statement, South Western said, "After crossing the railroad between Surbiton and Earlsfield, all London lines are now open.
"The trains that run through these stations return to normal, but some services can still be stopped or delayed by up to 40 minutes, with a break expected until 9:00 pm."
The railroad company added, "We used to be told by an intruder on the Surbiton-Earlsfield railroad, and the Network Rail and police staff have been helping people in a safe place.
"To ensure the safety of all involved, the power to the track had to be switched off, so that all routes between Surbiton and Earlsfield in the direction of London were blocked for about 40 minutes.
"The closure of the main line near London Waterloo has led to long train runs into and out of London Waterloo."
Services to and from Waterloo may be postponed, canceled or changed at short notice following the disruption.
South Western said that tickets will be accepted for the following services:
- London Underground between Wimbledon and London via a judicious route
- London buses between Surbiton / Hampton / Chessington South / Ewell West and London Waterloo
- Southern Railway Services between Epsom / Dorking and London Victoria
- GWR services between London Paddington and Exeter St Davids
- GWR services between Reading and Basingstoke
- GWR services between Redhill and Reading
- Cross-country skiing between Reading and Bournemouth via Basingstoke
For more information or referrals, passengers were urged to speak with the station staff.