An 82-year-old pensioner required hospital treatment after being tasered by police, a watchdog has said.
The elderly man was being arrested for carrying an offensive weapon and criminal damage when officers fired the electroshock weapon at him.
The Independent Police Complaints Commission has launched an investigation into the incident in Ealing, west London.
The taser was discharged after armed police were called to Islip Gardens on the morning of June 28 to reports of a man being threatened.
The 82-year-old was taken into custody at Acton police station but, when he complained of pain, officers called an ambulance and he was taken to Central Middlesex Hospital.
He remained in hospital for several days and has since been discharged, the IPCC said.