A masked mob of about 30 people attacked police officers with bottles after they were called to help a teenager who had been stabbed in east London.
The group threw bottles and other missiles at police in Chatsworth Road, Hackney at about 11.30pm last night.
An 18-year-old boy was found with serious stab wounds to his torso at the scene and was taken to hospital, police said.
His condition today was described by Scotland Yard as "serious but not life-threatening".
Residents spoke of their shock after witnessing the attack.
One Twitter user said: "Saw something horrible happen on Chatsworth Road earlier. Powerless to stop it. Feel crap."
Another wrote: "Just saw a lil boy get bottled on Chatsworth Road.. He was out of it on the floor all limp.. I really hope he survives! The kids of today.."
A second 18-year-old boy was being questioned in connection with the incident today after being arrested on suspicion of affray and possession of an offensive weapon.
In a statement, Scotland Yard said: "Officers at the scene were threatened by a large group of around 30 people. The group had covered their faces and threw bottles and other missiles at police. No officers were injured during this incident.
"One emergency response team officer pursued and apprehended a suspect in connection with this incident despite continuing threats from a volatile crowd."
Detectives in Hackney are appealing for witnesses to speak to them.
Anyone with information can contact Hackney Police on 101 or speak to Crimestoppers anonymously on 0800 555 111.
