A three-year-old girl was among those treated for injuries after men allegedly used what police believe was pepper spray during a suitcase robbery at Heathrow Airport.

The Metropolitan Police said the substance was sprayed when a group of four men robbed a woman's suitcase in a car park lift on Sunday.

Police said those in the vicinity were affected by the spray, with 21 treated for injuries, including five who were taken to the hospital.

A 31-year-old man was arrested on suspicion of assault, and other people were being searched for.

The Met said it was an isolated incident and was not being treated as terrorism or protest-related.

Commander Peter Stevens said police believe those directly involved were known to each other. The force earlier said there was an argument that escalated, resulting in people being injured.

Those injured are not believed to be in life-threatening or life-changing conditions.

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 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 BBC. "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 into 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 BBC he saw "armed police running around and checking people's vehicles" as he was attempting to leave the car park.

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 but resumed running later in the day.

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 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. Shuttles usually run every 15 minutes.

Police and Crime Minister Sarah Jones told people in the area to "follow police advice," which she said is being "regularly updated".

"My thoughts are with all those affected," she said in a post on X, expressing her "heartfelt thanks" to emergency services for their "rapid response".