Southern Rail users say they have missed flights as an overcrowded rail replacement service caused crowds to bottleneck at Redhill station.
Police were on the scene as frustrated crowds, unable to use the Gatwick Express service to the airport, continued to swell into Sunday evening as they waited for buses.
Southern Rail had advised during the week that it would be operating a bus replacement service for the Gatwick Express between Redhill and Gatwick Airport while "essential maintenance work" was carried out.
Melany Dominguez was among those who missed their flights and blamed poor planning and crowd management by the rail service.
She said she saw three police officers trying to manage the crowd which she estimated numbered in the thousands: "People were getting agitated."
By the time she managed to get onto a bus it took her to the wrong terminal, she said.
"I and a few young guys were running all the way to the airport to the terminal - I think they just made it. I was on the 5pm to Madrid so I just missed it."
She checked the next morning's flight on the bus, and it was £80, but by the time she missed her flight and went to rebook it was £95.
Ms Dominguez said she would be sleeping in the airport overnight, as the cost of the new flights left her unable to afford a hotel.
She hoped travellers who missed their flights would be compensated by the rail company.
"That would be ideal... I'm not really optimistic but I'm going to try. But if maybe all those people in there stick together we could say 'you need to sort this situation out'."
Cory Acasio told of witnessing chaotic scenes of frustrated commuters tripping over and pushing one another as the crowds grew.
"I stopped at the very front entrance to try and let an elderly person on, but no sooner did I make the gap that someone just barged right through, literally pushing me against the entrance."
He also saw passengers nearly trampling one another.
"Not sure if there were injuries as I was focused on moving forward, but I did look back and saw the people who tripped get up and some people did help, but most were just aggravated and focused on getting in the bus."
The crowds were a "nightmare" for passengers travelling with children, he said.
"I also saw a child screaming because she lost her mom, but luckily a stranger grabbed her and alerted her mom who was battling with getting through with a pram."
More travellers lashed out at Southern on social media, many venting their anger over Twitter, posting photographs of waiting crowds, and claiming to have missed flights due to the delays.
The rail service's Twitter account responded to some customers, saying they could apply for a refund for their Gatwick Express tickets.
National Rail and British Transport Police have been approached for comment.