Scotland Yard is facing a new case of alleged police racism after a black firefighter who tried to assist officers while off duty claimed he was targeted because of his skin colour, abused, assaulted and shot with a stun gun.

The complaint lodged by Edric Kennedy-Macfoy, 28, from north London, is the 10th case involving alleged racism known to have been referred to the official police watchdog in the last three weeks.

Kennedy-Macfoy was driving through Harrow around 3.30am on 4 September when he saw a young man hurl a rock at a police van. After noting a description of the young man, Kennedy-Macfoy, who also trained as a police constable, flagged down the van driver and approached a line of officers to pass the information on.

He said he was horrified at their response, which he alleges included officers behaving like "wild animals": swearing at him, dragging him from his car, subjecting him to a "violent" attack and eventually shooting him with a stun gun.

Kennedy-Macfoy was found not guilty in February of obstructing police. During a two-day trial at Brent magistrates court, Inspector David Bergum, who was present on the night, said his officers were in a "stressful" situation and had been dealing with a group of partygoers who had been throwing missiles at them.

He said of Kennedy-Macfoy: "I couldn't say he was anything to do with the party. The party was all black. He was black. He had driven through the cordon. I had to do a quick risk assessment."

Details of the inspector's comments in court were sent to the Metropolitan police's complaints department last week. The force confirmed four days later that it would refer the case to the Independent Police Complaints Commission (IPCC).

There are now 12 cases of alleged racism under investigation: six are being investigated internally at the Met, while the other six are being handled by the IPCC.

They include the case of Met police constable Alex MacFarlane, who discovered on Wednesday that he is to face criminal charges over a mobile phone recording which allegedly captured him racially abusing a black suspect.

Kennedy-Macfoy's complaint, which is against six police officers, brings the total number of Met officers currently under investigation for alleged racism to 26.

The firefighter, who left the Hendon police training academy after his mother became ill with cancer in 2005, said he hoped his complaint would make police "think twice" before stereotyping black men.

Although Kennedy-Macfoy has worked alongside police officers "almost every day" for six years, and has a number of close friends in the police, he said he had struggled to return to work as a firefighter since the experience.

On the night he was shot with the stun gun, he was wearing a three-piece pinstriped suit and driving a white Audi when he encountered a police roadblock and assumed there had been a road traffic accident.

In fact the police were dealing with the aftermath of a party that had been closed down, and officers had been subject to a hail of bottles and bricks. Kennedy-Macfoy had stopped behind a police van when he saw a young man pick up a rock.

"He launched it at this police van window and I was actually scared for whoever was in the police van," he said. "I knew it was occupied because the reverse lights kept on coming on and off."

The firefighter noted the man who had thrown the rock was black, wearing blue jeans, a black top and holding a red garment. When the van driver turned the vehicle, he tried to flag him down to pass on the description. According to Kennedy-Macfoy's complaint, the van driver did not let him speak, shouting: "Fuck off you prick."

When the van moved away, Kennedy-Macfoy drove to a cordon of police officers with shields further along the road. He said he pulled up by the officers for three reasons: to relay the description of the young man who had thrown the rock, request the badge number of the van driver who he alleged swore at him, so he could complain, and to ask for the quickest route home.

However, before he was able to convey the information, Kennedy-Macfoy alleges a number of officers began "hurling abuse" at him, charging at his car and grabbing him "viciously" through the windows.

He says he was dragged from the car and in the ensuing melee the officers repeatedly encircled him and shouted profanities. He said he replied calmly and showed his palms to the officers, telling them: "Listen guys, I haven't done anything wrong. I'm a firefighter – I work with you lot and I just want to explain something. I've showed no aggression toward any of you."

The fireman said he recalled police telling people who were filming the scene with their mobile phones: "Turn those fucking cameras off." He said the stun gun was discharged when he was walking backwards with his hands in the air.

In court, the police officer who discharged the weapon conceded he did not warn Kennedy-Macfoy he was about to be shot – an apparent contravention of Home Office guidelines.

Kennedy-Macfoy's complaint against police was submitted shortly after the attack. He alleged he was shot without warning and said he believed he had been targeted because of his race.

The Met should automatically refer all complaints involving the use of their Taser stun guns to the IPCC. However in Kennedy-Macfoy's case, the force decided to deal with the complaint internally for seven months. On Wednesday, following inquiries from the Guardian, and a letter from Kennedy-Macfoy's lawyers which drew attention to the evidence officers gave in court about race, the Met decided it would refer the case to the IPCC.

The Met said in a statement that it had not automatically referred the case to the IPCC due to "oversight". It added: "Such allegations are taken extremely seriously and the investigation will explore all the circumstances and evidence … As a Taser was discharged this case is a mandatory referral to the IPCC." The IPCC confirmed it had received the referral and was "assessing" the complaint.

Kennedy-Macfoy said he might not have pursued his case if police had not sought to prosecute him – a move he alleges required them to "concoct false accounts of events" to justify their actions.

"People make mistakes; you've got good cops and bad cops," he said. "People act differently under pressure. If at that point they had just said: 'Mate, so sorry – we have been dealing with this party and got it totally wrong,' I really wouldn't have pursued anything. I still would have been pissed off, but I would have accepted their apology and their acknowledgement that they had messed up."

If the attempt to prosecute Kennedy-Macfoy had succeeded he would have faced losing his job. He was acquitted on 7 February, despite the evidence against him given by several police officers.

Kennedy-Macfoy's solicitor, Shamik Dutta, of Bhatt Murphy solicitors, voiced concerns at his client's allegations, saying: "The question many people are bound to ask is why an off-duty firefighter, wearing a pinstriped suit and offering assistance to the police, should have been dragged from his car, shot with a Taser, locked up for many hours and then prosecuted for an offence he did not commit by the very officers he was trying to help.

"Our client now expects a comprehensive investigation which examines what role his race has had in the horrific events he has been forced to suffer."

Kennedy-Macfoy said this was not his first experience of discrimination at the hands of police: "I always get stopped by the police and it's always the same. [Police say:] 'Oh, you know, loads of these cars get stolen, so we just need to check you are who you say you are, blah blah blah.' And I know it is because I'm black. My friend Vince, he's a fireman – he borrows my car sometimes and it's a running joke at the fire station – he's never been stopped."

Brian Paddick, a former senior officer at Scotland Yard, said on Thursday he would appoint an independent commissioner for standards to help tackle "endemic" racism within the Metropolitan police if he were elected London mayor in May.

• This article was amended on 20 April 2012. The original article included an extraneous sentence about the IPCC referral, and also referred to five cases of alleged racism being handled by the IPCC instead of six. These errors have been corrected.

• Inspector Burgum denies any involvement in or responsibility for the abuse, assault or tasering of Edric Kennedy-Macfoy or that this attack was racially motivated. A further article was published on 25 November 2014 here.