Police were called to Ridley Road in Dalston at about 14:10 BST following reports of shots being fired.
Armed officers found a man in his 30s with a gunshot wound to his leg. He was taken to hospital and is said to be in a "serious but stable" condition.
Police searched the area and recovered a firearm. They also made 13 arrests for a variety of offences.
Ridley Road has been closed while the police carry out their investigation.
A man has been shot in the leg and 13 people arrested following an incident in east London.