A British Israeli academic has told the BBC of his shock at being detained by Israeli police for wearing a Jewish kippah, or head covering, embroidered with an Israeli and a Palestinian flag.

Alex Sinclair, 53, said he was taken from the cafe where he was sitting near his home in Modiin, central Israel, on Monday, ordered to hand over his kippah and locked in a cell.

He said when the item was returned to him, the part with the Palestinian flag had been cut off.

The unusual case has gained domestic and international attention after Sinclair shared the details in a social media post. Police told the BBC a complaint has been filed with their internal investigations division.

Sinclair, who is also a novelist, was working on his laptop in the cafe when, he said, "a religious man came over to me with an angry face and shouted at me that my kippah is against the law".

He said he invited the man to sit down to discuss his views, but he refused and said he would call the police. "Five minutes later, the police arrive," he wrote on Facebook. "Two officers, and they immediately tell me that my kippah is against the law and that they are going to confiscate it."

Sinclair said he tried to explain "politely" that his kippah was not illegal but was taken into detention and driven by police car to the police station. He said he was forced to hand over his possessions and unable to make a phone call. Sinclair said he was then frisked and locked in a cell.

Twenty minutes later, Sinclair said, he was told he could leave but without his kippah. He said when he insisted on it being given back, the officer handed it to him with the Palestinian flag cut out. Sinclair called the incident "surreal".

"That photo of the ripped kippah – there's something so kind of evocative about it," he reflected. "I think that's part of the reason that this story has gone so crazy."

In a statement, Israeli police said officers had attended the scene to "assess and address" a report on a hotline about a man wearing a kippah with a Palestinian flag.

"During the handling of the incident, the individual was brought to the police station where following clarification, the individual was subsequently released. As a complaint has been filed with the Police Internal Investigations Division within the Ministry of Justice, no further details can be provided at this stage."

There is no explicit Israeli law banning public displays of the Palestinian flag. While Israeli courts have viewed it as a protected form of expression, Israeli police are authorised to remove or confiscate them if they are deemed "a threat to public order" or identifying with a terrorist organisation.

The current far-right National Security minister, Itamar Ben-Gvir, has instructed police to clamp down on Palestinian flags, in a way that Israeli rights groups have said is illegal.

Sinclair regularly wore his kippah with the Israeli and Palestinian flags on a black background over the past 20 years, after he specially ordered it from a shop in Jerusalem. He described it as a symbol of "the messy ambivalence of my Jewish-Zionist identity".

"I'm doing all of this as a Zionist, as somebody who chooses to live here, as somebody who believes in the right of Israel to exist and to flourish in security, along with the Palestinians having those same rights as well," he told the BBC. "I've not given up on a future where we can live together in peace and security."

Sinclair said he chose the design of his head covering to distinguish himself from right-wing and far-right religious nationalists. "When you walk around Israel and people see you in a kippah, they immediately associate you with certain political and religious groups who I don't want to be associated with to put it mildly," he said.

"The journey behind it was just trying to figure out a way to keep a kippah on my head. How do I take part in a Jewish ritual that is meaningful to me but do so in a way that feels authentic to me."

Sinclair, an observant Masorti or conservative Jew, who grew up in north London, said he has often had positive reactions and "moving moments" in response to his choice of kippah from Palestinian citizens of Israel.

He acknowledged that he has had "some less pleasant moments" too but said that he previously managed to engage people in "interesting conversations about politics".

After this week's experience he said he felt "anger and frustration as well as concern" that he was now on the police radar.

The leader of Israel's Democrats Party, Yair Golan, criticised the police over what had happened.

"This isn't just a story about a kippah that was crudely torn off by police. It's a story about the collapse of the Israeli police," he wrote on X.

The Hebrew University in Jerusalem – where Sinclair works as a lecturer in Jewish education – has written a strongly-worded letter to the Israel police force.

It said it "is troubled by the blatant violation of freedom of expression in the public sphere and strongly condemns the conduct of the police officers during the incident, in the course of which they cut Dr Sinclair's kippah".

In his complaint to the Department for Internal Police Investigations, Sinclair has claimed unlawful detention and damage to property. He has asked for compensation for his ruined kippah.

He is also planning to order a new kippah with both flags, and added "some people are saying that maybe it'll start a trend".