A three-year-old girl is among those who were treated by ambulance crews after a group of men used pepper spray on a woman they had robbed of her suitcase at Heathrow Airport.
Officers were called shortly after 8am to reports that multiple people had been assaulted by a group of men at the multi-storey car park at Terminal 3.
A 31-year-old man remains in custody and enquiries remain ongoing to locate further suspects.
The London Ambulance Service treated 21 people with injuries, with five taken to hospital.
Commander Peter Stevens said: "Our team have been working at pace today to review CCTV from the area and speak with witnesses at the scene.
"At this stage, it’s understood that a woman was robbed of her suitcase by a group of four men, who sprayed a substance believed to be pepper spray in her direction.
"This occurred within a car park lift, with those in the lift and surrounding area affected by the spray.
"Our officers are working to determine the full circumstances around what happened but we do believe this to be an isolated incident with those directly involved known to each other."
An emergency vehicle outside Heathrow Terminal 3 after a man has been arrested on suspicion of assault
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He continued: "We appreciate that this has been a concerning incident and I commend the actions of the emergency services and members of staff at Heathrow Airport."
Eyewitness Tom Bate told GB News: "It was incredibly eerie because one at a time, everyone, including me, started coughing all at once. It felt like there was a burning at the back of our throats.
"There were children, there were elderly people...everyone just coughing and spluttering,"
By midday, dozens of passengers were waiting at the terminal’s bus stop for shuttles to the long-stay car park and elsewhere.
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Armed police rush to Heathrow Airport after multiple people assaulted in 'pepper spray attack'One family, who did not wish to be named, said they had waited for three hours at the bus stop after their flight landed at around 8am.
Heathrow staff handed out bottled water to waiting passengers on Sunday afternoon.
A Heathrow spokesman said: "The shuttle buses to the long-stay car park were impacted by congestion on the roads following an earlier incident which saw the Central Terminal Area tunnel closed for safety reasons.
"The buses are now operating as normal and we apologise for any inconvenience caused."
Emergency vehicles outside Heathrow Terminal 3
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A spokesman from the London Ambulance Service said: "We were called today at 8.04am to reports of an incident at Heathrow Airport.
"We sent a number of resources to the scene, and cared for 21 patients. Five patients were taken to hospital.
"The ambulance service was stood down from the scene at 10.58am."