LONDON — Twenty people have been injured, two of them seriously, as a busy commuter train from London derailed on Thursday near the southeast English town of Lewes, emergency services said.The South East Coast Ambulance Service said there were around 150 passengers on board the train which departed from London Victoria station and veered off the tracks shortly before 4:00 p.m. (1500 GMT).Ian Drummond-Smith from the British Transport Police said a “major incident” had been declared but that all passengers on the train had been “safely evacuated.”Footage from the scene in East Sussex showed at least three overturned carriages and paramedics on site. The cause of the incident remained unknown.The accident involved a train from Southern Rail, which connects areas south of London to the capital.Photographs from the scene showed people standing on top of overturned carriages, with passengers describing "pandemonium" on board and screaming.British Transport Police said three carriages derailed, leaving several people trapped. All 150 on board have now been evacuated.Several rail routes in the area were closed, as emergency teams rushed to the scene and people were advised to avoid the area.Patients were assessed and treated at the scene, with some sent to local hospitals.British Transport Police Chief Superintendent Dominique Ioannou said in a press briefing on Thursday night that a number of passengers were initially trapped but were all safely evacuated from the train."The train started rocking violently and then I remember clocking eyes with this guy and then there was a lot of screaming," passenger Rachel Heath said."It felt like we flew in the air and the carriage landed on its side and it slid. I felt like it was going to completely flip over.South East Coast Ambulance Service said passengers who did not require hospital treatment had been taken to a local community center.Outside the center, Yana Ludwig, who was travelling alone on the train from London to Lewes, said the experience as "completely terrifying"."There was a banging. Everything just went topsy turvy," the 56-year-old from the United States said."It seemed like a really long time but it probably was 10 seconds of that before the carriage actually fell on the side."She said she was "pretty banged up", adding that everyone in the train was checking on their fellow passengers.Network Rail said it would be "inappropriate" to comment as to what caused the derailment."I would like to reassure everyone that we are working closely with the Rail Accident Investigation Branch, who are on site to establish exactly what has happened," said Lucy McAuliffe from, Network Rail.BTP assistant chief constable Ian Drummond-Smith said: "We've declared a major incident and a significant emergency service response is ongoing at the scene, but all passengers on the train have now been safely evacuated from the service.""This remains a complex and ongoing multi-agency response," a spokesperson said.Fire engines, ambulances and police were stationed on Landport Road, in Lewes, alongside a double decker bus by the community hub waiting for passengers to board.East Sussex Fire and Rescue Service said it sent 10 fire engines, a command support unit, technical rescue teams and other resources to the scene.Network Rail's McAuliffe said no timetable could be given for when the railway would be able to reopen after the derailment.She added: "We will provide updates as soon as we have a clear understanding of the recovery work required and when the railway can safely reopen."Southern Rail said major disruption was expected to continue until further notice. It advised passengers to check before they travel.Tickets were being accepted on alternative Southern, Thameslink and Gatwick Express services with rail replacement buses arranged for travel to Lewes and to Haywards Heath.The railway operator advised those travelling to Gatwick to allow "at least an extra 150 minutes for your journey" or use an alternative route.Transport Secretary Heidi Alexander said she was "deeply concerned".She wrote on X: "Grateful to emergency services who are on the scene supporting those impacted."We are working quickly with the rail industry and local partners to assist passengers."Train accidents, particularly fatal ones, are relatively rare in Britain.A train driver was killed and dozens of people injured when two trains collided north of London in June.Four people suffered minor injuries in a train derailment due to a reported landslide in remote northwestern England last November.