Unarmed African-American tasered and shot dead by police after his sister called officers for help.

A police officer in the Californian city of San Diego shot and killed a mentally ill black man who was unarmed after his sister called authorities for assistance when he began acting strangely.

Police from the El Cajon suburb released a statement late on Tuesday night, several hours after the shooting outside the Broadway Village shopping centre, confirming the man died in a hospital. His family identified him as Alfred Olango.

Police were called by Olango’s sister who said he was acting strangely and not himself. The aftermath of the fatal shooting was filmed by a bystander who posted the clip on Facebook. That video has been viewed about 40,000 times.

“Why couldn’t you tase him? I told you he is sick – and you guys shot him!” Olango’s sister can be heard telling officers in the video. “I called police to help him, not to kill him.”

Jeff Davis, the El Cajon police chief, said the man was not armed, adding Olango ignored calls to remove his hands from his pockets and pulled out an object.

Davis said one officer used a stun-gun against him while another fired his weapon because 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.”

Witnesses said they heard five shots as police confronted Olango.

Davis said the two officers involved in the shooting, each with more than 20 years of service, had been placed on administrative leave pending the investigation.

Local news agency CBS8 reported that “several witnesses alleged that the officers were unduly quick to open fire and suggested that their actions had been influenced by the fact that they were dealing with a black man, one they described as mentally challenged”.

“One man angrily told reporters at the news conference that the victim was suffering a seizure prior to the shooting, and another described seeing him with his hands raised at the moment the shots sounded,” CBS8 said.

The shooting spurred protests at the scene for several hours, alleging police racism. Demonstrations were also later held outside the police department.

It released a photo taken from a mobile phone of the moment. Police said an object had been recovered from the scene, but did not say what that object was.

According to Mapping Police Violence , Olango became the 217th black American to be killed by police so far this year.

The deaths of black men at the hands of police have spurred protests across the US, most recently last week in the North Carolina city of Charlotte.

The fatal shooting of Keith Lamont Scott, 43, triggered days of unrest, forcing the governor to declare a state of emergency and deploy the National Guard.