The police union representing a Florida officer who shot an unarmed black carer while he was lying on the ground with his hands in the air has defended the shooting – saying the officer was actually aiming for his autistic patient.

Charles Kinsey was trying to take his 23-year-old patient back to a mental health centre in North Miami when he was surrounded by armed police, who had been called to reports of a man with a gun threatening to shoot himself.

A video of the incident on Monday (18 July), taken by a bystander, showed Kinsey pleading with officers from North Miami Police Department not to shoot as he lay down and told his patient sitting next to him to be still.

Moments later Kinsey was shot in the leg by one of the officers despite still holding his hands in the air.

John Rivera, representing the officer who fired on Kinsey as head of Miami-Dade's Police Benevolent Association, said in a press conference on Thursday: "The movement of the white individual [the autistic patient] looked like he was getting ready to discharge a firearm on Mr Kinsey.

"The officer discharged, trying to strike and stop the white male, and unfortunately he missed the white male and shot Mr Kinsey by accident."

He added: "This is not a case of a rogue cop, or a case of police abuse. This is a case where a police officer was trying to save Mr Kinsey's life and unfortunately his shot went astray."

North Miami Police Department says the incident occurred after officers were called to reports of an armed man threatening to kill himself at the junction of Northeast 127th Street and 14th Avenue.

They instead found Kinsey speaking with his autistic patient, who had wandered off from a mental health centre and was sitting down in the middle of the road to play with a toy truck.

Kinsey, 47, can be heard shouting at police before he was shot that his patient is not carrying a firearm, but instead a toy truck. He says: "All he has is a toy truck. A toy truck. I am a behaviour therapist at a group home."

Rivera said the video of the incident "poorly" portrayed the events, saying it was filmed "from a different vantage point and much closer than the officer was". He claimed officers couldn't hear what Kinsey was saying.

The officer who fired on Kinsey, identified only as a 30-year-old Hispanic man with four years' service, has been placed on administrative leave, as is standard departmental policy.

He said in a written statement: "I took this job to save lives and help people. I did what I had to do in a split-second to accomplish that, and hate to hear others paint me as something I'm not."

North Miami Police Chief Gary Eugene said an investigation into the incident had been turned over to the Miami-Dade State Attorney's Office, and urged any witnesses to come forward. He confirmed no firearm was found at the scene.

On Thursday, US Rep Frederica Wilson, congresswoman from the 24th district of Florida, said she was "shocked and stunned" at the shooting, saying it was "like a nightmare".

She said: "This is not supposed to be happening in North Miami. North Miami is a city where the police officers and the community gel. So many of our police officers come from the community, live in the community, work with the community.

"The video is like a nightmare. I can't believe it [and] I've seen several police shootings. We say to our boys and our men, 'If you're ever stopped by the police, freeze – don't move'. That's number one on the brochure that we created to train the boys.

"What else could we have told him [Kinsey]? What could have saved him from being shot? From what I saw, he was lying on the ground, with his hands up, freezing. And he was still shot."

As a small group of protesters gathered inside the North Miami Police Department HQ demanding answers, Eugene told the media a "transparent" investigation would take place, adding: "I can assure you our department is committed to keeping the community safe."

Kinsey is recovering in hospital with his injuries not thought to be life-threatening. He is expected to be released this week.