Protesters gathered on Wednesday in a California town following the police shooting of an unarmed black man who was reportedly mentally ill.

The man, identified by a relative as Ugandan native Alfred Olango, 30, was shot on Tuesday in the San Diego suburb of El Cajon after police received a call about a man behaving erratically and walking in traffic.

El Cajon police chief Jeff Davis said Olango had ignored repeated calls by officers to remove his hand from his pocket and one officer used a Taser against him while another fired his weapon as they felt threatened.

'At one point, the subject rapidly drew an object from his front pants pocket, placed both hands together and extended them rapidly toward the officer taking up what appeared to be a shooting stance,' Davis said in a statement.

'At this time, the officer with the electronic control device discharged his weapon.

'Simultaneously, the officer with the firearm discharged his weapon several times, striking the subject.'

The shooting took place as the United States reels from a string of police killings of black men that have raised racial tensions nationwide.

One woman posted a Facebook Live video after the shooting that showed a distraught woman who identified herself as Olango's sister and said she had called police to help her brother who was mentally ill.

'You guys came and killed my brother,' the woman wails in the video that had been viewed 82,000 times by Wednesday morning. 'I called you guys to help my brother. You killed my brother in front of me.'

Witnesses said they heard five shots as police confronted Olango.

IMAGE: Agnes Hasam, a family friend of the Alfred Olango, speaks to protesters gathered at the El Cajon Police Department headquarters to protest fatal shooting of an unarmed black man on Tuesday by officers in El Cajon, California. Photograph: Earnie Grafton/Reuters