A man has been arrested on suspicion of assault after a number of people were sprayed with what is believed to be pepper spray at a Heathrow Airport car park, police said.
A group of people who knew each other were involved in an argument which escalated and resulted in people being injured, police said, with other suspects being searched for.
Twenty-one people were treated for injuries with five of those taken to hospital.
The Metropolitan Police said it was an isolated incident and was not being treated as terrorism or protest-related.
Earlier in the day trains were delayed and the car park was closed due to the incident but it reopened by 11:30 GMT.
Police and emergency services were called at 08:11 to the multi-storey car park at Terminal 3.
Tom Bate was waiting for a lift in the Terminal 3 car park when he saw “young men dressed in black darting through the crowd” before going through a side door.
“As soon as they left people started coughing, and then I started to cough,” he told the . “I felt a burning at the back of my throat.”
Driving out of the car park, he said he saw police arriving. “It was an amazing thing to see the police running to danger,” he said. “I’m so glad to hear now that it’s not terrorism because it felt like I was in the middle of an attack – it was pretty intense.”
Social media footage showed a large armed police response, fire engines and an officer appearing to search a handcuffed individual.
Fin Fraser, who flew into Heathrow from the US on Sunday morning, told the he saw “armed police running around and checking people’s vehicles” as he was attempting to leave the car park.
“When they came to ours, they knocked on our window and told us to wind it down and then they looked inside the car.”
Describing it as a “surreal experience,” he added: “It felt like I was in a James Bond movie”.
There was disruption to rail services on Sunday, with Heathrow advising passengers to “allow extra time when travelling to the airport”.
The Heathrow Express and parts of the Elizabeth Line were suspended on Sunday morning while responders attended the scene but resumed running later in the morning.
Heathrow has advised passengers to “allow extra time when travelling to the airport and to check with their airline for any queries”. Due to congestion on surrounding roads following the incident, the airport has advised passengers to use public transport.
Airport staff handed bottles of water to passengers waiting for buses from Terminal 3, where one family told PA news agency they had been waiting at the bus stop for three hours for a shuttle to the long-stay car park after their flight. Shuttles usually run every 15 minutes.