Cops rushed to Kingsland High Street in Dalston, Hackney, north-east London, at around 9.20pm following reports of gunshots.
Officers, including specialist firearms officers, and London Ambulance Service attended.
Three adults and a child were found with gunshot injuries at the scene, the Metropolitan Police said.
They were all taken to an east London hospital.
The child remains in a serious condition and police are awaiting further details of the other victims’ conditions.
Cops are working to establish the circumstances of the incident.
No arrests have been made at this time.
Footage shared on social media showed multiple police and ambulances at the scene.
Dalston Kingsland station was shut due to a “police investigation”.
Witnesses claimed the police presence was related to a drive-by shooting.
The owner of Dalston Jazz Bar told The Sun: “People were running round the corner screaming.
“They shut everywhere down. It was a guy on a motorbike doing the shooting.
“Police saw a helmet and started crowding around it because they thought it might be his.”
The alleged shooting involved a motorcyclist aiming and firing a gun towards the direction of Evin Restaurant.
One anonymous diner said he used his coat to stop a man’s leg bleeding.
“There was a big group having dinner in the rear,” he told the Hackney Gazette.
“I couldn’t get my usual table so got one close to the front window.
“The big group paid up and left and shortly afterwards that’s when the loud bangs – which were obviously gunshots – happened.”
Deputy Assistant Commissioner Matt Ward said: “We know Londoners will be shocked by what has taken place tonight.
“Our thoughts go to all those affected. Fast moving enquiries are under way and we will update as soon as we can.
“If anyone has any information, please contact us.”
A London Ambulance Service spokesperson said: “We were called at 9.21pm on Wednesday 29 May to reports of a shooting on Kingsland High Street, Hackney.
“We sent resources to the scene, including ambulance crews, advanced paramedics, incident response officers and members of our tactical response unit.
“We also dispatched a trauma team from London Air Ambulance, which consisted of a doctor and a paramedic in a car.
“We treated three adults and one child at the scene before taking them all to a major trauma centre.”
Anyone with information that can help police should call 101 or post @MetCC reference CAD 8082/29May.
To remain anonymous call Crimestoppers on 0800 555 111.