Two police officers are being treated in hospital after being stabbed while on duty in north London.
They were responding to reports of a disturbance at a chemist in Church Street, Stoke Newington, when one of the officers was stabbed in the thigh.
The other policeman suffered superficial injuries and both are currently in hospital.
Police said a man in his 20s was arrested soon after and was being held at a north London police station.
Armed officers and police helicopters were drafted in after the attack and a knife has been recovered.
The second officer, aged in his 20s, suffered his minor injuries when trying to stop the attacker from escaping, police said.
Both officers are described as being in a comfortable condition in hospital.
After the attack, the man ran into a nearby cemetery and a large-scale search was launched.
The area has been cordoned off to allow forensic experts to examine the area.
Superintendent Leroy Logan said a thorough search of the area was being carried out and they were not discounting the possibility of any other weapons being involved.
Police have appealed for anyone who has information about the incident, which happened at about 1100 BST on Saturday, to contact them.