An unarmed disabled black man was shot dead by police in San Diego after his sister called 911 to ask for help, reports suggest.

Alfred Olango, 30, was gunned down in a parking lot on Tuesday afternoon, triggering more protests against police brutality after a spate of deadly shootings at the hands of cops.

Just minutes after the fatal shooting, a witness captured his horrified sister screaming: 'I called police to help him, not to kill him'.

Video of the shooting appears to show Olango taking something from his pocket and pointing it at officers, but police have refused to said what it was. They did not find a gun at the scene.

El Cajon Police Chief Jeff Davis said his officers fired because Olango quickly extended the object and took a stance 'suggesting he was about to shoot'.

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The photo above shows the moment right before Alfred Olango, 30, was shot dead by El Cajon police. One officer points a gun while another aims a Taser at the man

Police said they were called to the Broadway Village Shopping Center in El Cajon, after they received calls Olango was behaving erratically and 'not acting like himself', a statement from police reads.

Davis said on Tuesday night that the shooting victim was walking in traffic 'not only endangering himself, but motorists.'

He said the man refused multiple instructions by the first officer on the scene to remove his hand from his pocket.

But a witness told NBC 7 that Olango, who is believed to have suffered from mental health problems, had his hands up when he was fatally shot.

Police insist he did not have his hands in the air.

Maria Medina, an employee at the nearby Los Panchos restaurant in El Cajon, said one of her co-workers recorded video of the shooting, but then had her phone taken by police.

El Cajon cops denied any phones were 'confiscated'.

When the man did not follow their demands, one officer drew his firearm and pointed it the man

A second officer who responded to the scene drew a Taser, aiming it at the man.

'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,' the statement reads.

A video shared to social media by Rumbie Mubaiwa shows a woman identified as the victim's sister, Sarah, crying hysterically in the parking lot after witnessing the shooting

El Cajon Police say they were called to a parking lot (above) at the Broadway Village Shopping Center at 1pm after being told a black man was acting 'erratically'

The shooting victim's sister (above) can be heard saying: 'I called you to help me but you killed my brother,' who she named as Alfred Olango in the Facebook video

'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.'

Police provided first aid for the man at the scene until medics arrived and took him to the hospital.

He died from injuries a short time later.

It is unclear how many times he was shot or what part of his body was struck.

Davis said the second officer fired his Taser at the man the same time the other officer fired from his gun.

El Cajon Police say the man did not comply with orders from an officer and the encounter ended with an officer-involved shooting (scene pictured above)

The man was taken to Sharp Memorial Hospital, where he was pronounced dead. Shoes and medical supplies are pictured at the scene

A photo released by police shows the moment right before the man was shot dead by an officer, as another officer aims a Taser at the shooting victim.

The entire shooting was captured on a cell phone, but the video will not be released as it is part of an ongoing investigation, said Davis.

A video shared to Facebook by Rumbie Mubaiwa shows a woman named Sarah, who said she was the dead man's sister, crying hysterically in the parking lot after witnessing her brother being shot.

Sarah, who said she called police three different times, can be heard in the video, saying: 'I called you to help me but you killed my brother.

'Why couldn't you guys tase him? Why why why why?'

Police officers can be seen approaching the woman in the video to question her.

'I called so many times to help him,' she said before telling a female police officer that her brother's name is Alfred Olango.

The video posted to Facebook by Mubaiwa did not capture the actual shooting.

Meanwhile tennis star Serena Williams has said she 'won't be silent' about the killing of black men by police officers.

Williams posted on Facebook on Tuesday she was worried for members of her family and she quoted Martin Luther King Jr., saying 'there comes a time when silence is betrayal', adding 'I Won't Be Silent.'

Witnesses claim to have heard police fire several shots at the man

It is unclear how many times he was shot or what part of his body was struck. Witnesses at the scene of the shooting (pictured) questioned the motives of the officers

Not long after the shooting Lieutenant Rob Ransweiler said: 'I hope the community of El Cajon allows for the details of the investigation to be released.

'I'm confident that the community will support the decision made by the officer.'

Authorities say the District Attorney's office along with the homicide division of the El Cajon police department will be investigating the shooting, CBS 8 reported.

The El Cajon Police Department has placed the two officers involved on administrative leave. They both have 21 years of service as police officers.

Witnesses at the scene of the shooting questioned the motives of the officers in the shooting, as people gathered and started demanding to get answers from the El Cajon police.

People gathered at the Broadway Village Shopping Center Thursday afternoon and started demanding to get answers from the El Cajon police about the incident (above)

Dozens of people who gathered at the strip mall where the shooting happened began chanting 'Black Lives Matter' and 'hands up, don't shoot'

Pastor Russell Bowman, center, prays out loud as people gather at the scene where the man was shot on Tuesday in El Cajon, east of San Diego

A woman who claims to have witnessed the shooting said the man was unarmed and was mentally challenged.

Author and film-maker Diane Sherlock tweeted: 'Police scanner is very clear it was called in as a 5150 (mental health) #AlfredOlango.'

Another witness, identified only as George, claims the police fired the shots at the man without any words being said.

'I didn't hear any command 'Halt', 'Stop' or 'I'll shoot,' George told NBC San Diego.

'I didn't hear any command or yelling. I didn't hear the man say anything. Next thing I see 'Pow, pow, pow, pow, pow' – five shots,' he added.

Maria Medina, an employee at the Los Panchos restaurant in El Cajon, which is located about 15 miles east of San Diego, said she was working when she heard the shooting.

Author and film-maker Diane Sherlock claimed police should have known there were mental health issues involved

She said officers came into the restaurant and confiscated everyone's cell phone after the shooting.

Ms Medina said one of her co-workers had recorded video of the shooting on her phone, which was confiscated by police.

But El Cajon police denied any phones were 'confiscated'.

The police statement reads: 'While detectives where on scene investigating the officer involved shooting, a witness came forward and notified officers they had video footage of the incident.

'That witness voluntarily provided their phone to the police department and gave written consent for the officers to view the video.'

A man points at police as he and others yell at police while at the scene where a black man was shot by police earlier in El Cajon

The El Cajon Police Department tweeted the message above about the shooting on Tuesday

San Diego City Councilwoman Myrtle Cole tweeted this message before news came in that the man had died

Police said that phone was the 'only phone provided to officers in this investigation' and they were reviewing the video and other videos recovered from the scene.

Officials said 'all video recovered so far in this investigation clearly shows the incident' as described by El Cajon police.

Some witnesses claimed the man had his hands up in the air at the time of the shooting, but El Cajon police tweeted: 'The investigation just started, but based on the video voluntarily provided by a witness, the subject did NOT have his hands up in the air.'

Dozens of people who gathered at the strip mall where the shooting happened chanted 'Black Lives Matter' and 'Hands up, don't shoot.'

In May, El Cajon City Council members approved the purchase of 88 body cameras.

Police Chief Davis said he was hoping to have the cameras in use by the start of 2017.

Before it was announced the man had died, San Diego City Councilwoman Myrtle Cole tweeted: 'My heart goes out to the family & loved ones of the victim at the El Cajon shooting today. I pray that he survives.'

The hashtag '#AlfredOlango' began circulating on Twitter last night as many users wrote messages of condolences and frustration over another black man being shot by police.

The shooting in El Cajon comes just three days after the video showing the September 20 police shooting of Keith Lamont Scott in Charlotte, North Carolina was released.

A police officer in Tulsa, Oklahoma shot and killed 40-year-old Terence Crutcher six days after Scott's shooting.

In the Tulsa shooting, officer Betty Shelby has been charged with manslaughter and if convicted, she faces up to life in prison.

Crutcher's and Scott's deaths have sparked protests across the country, as people question the law enforcement officers' actions.

Serena Williams posted on Facebook on Tuesday that she 'won't be silent' on the issue of black people being killed by police

The American Civil Liberties Union of San Diego and Imperial Counties released a statement on the El Cajon shooting.

' The ACLU of San Diego & Imperial Counties was concerned to learn of the shooting of a Black man by police officers in El Cajon earlier this afternoon,' the statement reads.

'It is too early to know many of the details of the actual shooting and what preceded it, and we hope that the El Cajon Police Department and the San Diego District Attorney provide the public with answers as quickly as possible, with transparency and accountability for all involved.

'Unfortunately, there are disturbing reports from a number of witnesses that police officers confiscated cell phones from people who witnessed the shooting.

Confiscating cell phones is a violation of the Fourth Amendment (unreasonable seizure without warrant or exigent circumstance) and the First Amendment (interference with the right to record in public) under the U.S. Constitution and analogous rights under the California Constitution.

'It is hard to see any kind of Fourth Amendment exigent circumstances at issue here.

The First Amendment issues are also significant, because by seizing phones, police would likely be preventing the dissemination of video captured by bystanders.'