Southern rail passengers say they missed flights and faced "dangerous" scenes and "stampedes" because of an overcrowded rail replacement service.
Police were called in as hundreds of people queued for buses at Redhill station to replace the Gatwick Express on Sunday.
The RMT renewed calls for Southern to be stripped of its franchise after the "terrifying pictures" emerged.
Southern apologised and said it would review the situation in future.
Local MP Crispin Blunt tweeted there was a "dangerous shambles" on the stairs at the station before passengers could even join the "shocking queue of people".
The service between Redhill and Three Bridges had been suspended due to planned engineering works.
Southern had warned in the run-up to the weekend buses would replace the Gatwick Express service from Redhill to the airport while "essential maintenance" was carried out.
But by 10:00 GMT on Sunday, Southern was forced to draft in additional buses as it struggled to cope with demand.
⚠️ Due to overcrowding between London Victoria and #Redhill services are subject to disruption. We have sourced additional bus replacement services to help ease the volume of passengers.— Southern (@SouthernRailUK) February 25, 2018
Even with extra buses, queues continued to build and passenger Cory Acasio described chaotic scenes of frustrated commuters tripping over and pushing one another as the crowds grew.
He said: "I saw a child screaming because she lost her mum, but luckily a stranger grabbed her and alerted her mum who was battling with getting through with a pram."
The Reigate, Redhill and District Rail Users Association said it had already warned Southern its plans would be inadequate ahead of the weekend.
It said: "Sadly, despite replying to our emails they didn't listen or take any further action."
The travel chaos marks the first of three Sundays where the track modernisation work on the Brighton Main Line will halt all Gatwick Airport and south coast-bound passengers.
The group's chairman Stephen Trigg has now written to Southern calling for the company and Network Rail to cancel the next two weekends' work until "you can work together to find a proper and safe solution", adding the situation at Redhill "is extremely dangerous".
Passenger Melany Dominguez missed her flight from Gatwick. She said she saw three police officers trying to manage the crowd, which she estimated to be "in the thousands".
"People were getting agitated," she added.
Rail user Justin Stevens tweeted: "Poor organisation. No communication between bus stewards. No queue system set up. Transport police stood there with hands in pockets. U created stampedes putting people at risk. Shameful."
RMT General Secretary Mick Cash said the "terrifying pictures" from Redhill showed "public safety is of little or no concern" to Southern and called on the company to be stripped of its franchise "before there's a major tragedy".
In a statement, Southern apologised and said it would "take a higher capacity requirement into consideration".
⚠️ We apologise for the disruption to our services today which resulted in long queues and wait times for replacement buses at #Redhill. We are very sorry for the distress this must have caused to our passengers.— Southern (@SouthernRailUK) February 25, 2018
It added: "Demand for the rail replacement service was very high...
"We secured additional bus services throughout the day to help to get passengers on the move, with some services added in the afternoon specifically offering a shorter Redhill to Gatwick route...
"We had buses scheduled to run every five minutes until the end of service and had additional staff at the station to help those waiting for them."
The firm said other rail operators had also been accepting its tickets.