“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.”
Armed officers arrested a 31-year-old man on suspicion of assault after responding to the scene.
“He remains in custody and inquiries remain ongoing to locate further suspects,” police said in an update.
Emergency responders treated 21 patients, with five taken to hospital, the London Ambulance Service said.
The 3-year-old received treatment at the scene, with all the injuries “not believed to be life-changing or life-threatening”, according to police.
Firefighters provided “specialist assistance” at the scene, London’s Fire Brigade said.
Terminal 3 at Europe’s busiest airport remained open throughout, but the incident prompted severe traffic and public transport disruption in the area, according to officials and reports.
Passengers complained about having missed flights due to its impact.
“We were literally stuck for an hour-and-a-half,” Jayesh Patel, whose family were headed to the airport for a flight to India, told AFP.
“We ran to the gate, and we missed the check-in by three minutes, and we were turned away.
“So we’re gonna have to drive 100 miles [160km] back home.”
-Agence France-Presse