A 12-year-old boy with a stab wound has been taken to hospital by air ambulance.
Police were called to Mayfair Avenue in Pitsea, Essex, at about 13:55 BST to a reported assault.
The boy had a knife wound to his back and was taken to the Royal London Hospital. His injury was described as serious but not life-threatening.
A girl and three boys, all aged 14, have been arrested on suspicion of causing grievous bodily harm.
They remain in custody for questioning.
Det Insp Stewart Eastbrook said he believed the people involved knew each other and officers were talking to witnesses near the scene of the attack.
“There were a number of people in the area at the time and we’d like to speak to anyone who saw what happened or has any mobile phone or CCTV footage that could help us with our enquiries,” he added.