More than 20 people have been detained by Turkish security forces for wearing a white T-shirt with the word "HERO" written on it in block letters.

In mid-July, chaos broke out in a courtroom in southwest Turkey when Gokhan Guclu -- a former soldier accused of his connections to an assassination attempt on President Tayyip Recep Erdogan during the 2016 failed military coup -- turned up to a court hearing in one of the T-shirts, prompting the session to be adjourned.

Guclu is also accused of belonging to what the Turkish government refers to as the Fethullah Gulen Terrorist Organization (FETO). Gulen , the reclusive Muslim cleric accused by Turkey of hatching the military coup attempt, has said that his supporters could have been involved in the coup attempt but has always denied any direct connection.

An investigation into Guclu -- and his choice of courtroom clothing -- was soon launched.

Guclu's sister sent him the T-shirt while he was in jail awaiting trial and prison authorities allowed Guclu to wear it to the hearing, according to state news media Anadolu.

The prosecutor's office is now investigating Guclu's sister and the government agencies responsible for running the prison where Guclu is incarcerated, according to Anadolu

Darbeci askerin 'hero' tişörtüne böyle yanıt verdiler https://t.co/upgkazBzVT pic.twitter.com/jmCYEGNQ0c — CNN Türk (@cnnturk) July 14, 2017

The day following Guclu's court appearance, a crowd of Erdogan supporters gathered outside the courthouse where they hung another white T-shirt embroidered with the word "TRAITOR" on it from a noose.

Political protest or fashion faux paux?

The "HERO" T-shirts were originally sold by a Turkish clothing store ubiquitous in shopping centers throughout the country. The brand has since halted sales of the now-controversial top, according to Turkish daily Cumhuriyet.

More than 20 people have been detained in Turkey for wearing the T-shirt, according to Turkish media reports. While most have been released, at least one person has been arrested after he tried entering a courtroom wearing it, according to Anadolu.

Two university students who were detained told police that they bought the T-shirts not knowing they were associated with the FETO trials, according to Anadolu.

Speaking to police, the two students said: "We don't follow the news. We bought the T-shirts at a shopping mall. We didn't know they were forbidden. We wanted to be matching."

Antalya'da 'Hero' tişörtü giydikleri için gözaltına alınan iki genç hakkında yeni gelişme https://t.co/zEQA4Y62fX pic.twitter.com/FJBmyCdzq5 — CNN Türk (@cnnturk) July 22, 2017

A hotel worker in the coastal town of Antalya who was also told police he had seen media reports about the controversy but hadn't given it much thought before choosing the top -- saying he just liked white T-shirts, according to Anadolu.

It's unclear what basis there is for detaining people for wearing a T-shirt under Turkish penal law.

Lawyer Gulan Cavli told CNN that wearing the T-shirt could only be considered a crime if it bore the official symbol of a recognized terrorist organization. But in the case of the "HERO" top, there are no legal grounds for detention.

"It is completely outside the law," Cavli said.

Guantanamo-style uniforms

In mid-July, Turkey's Justice Minister Bekir Bozdag announced a new rule that prohibits suspects from appearing in court wearing anything that has text on it.

"People will not be allowed to wear in court anything that has a picture, sign, text or any sort of message from now on. It is a rule now," Bozdag said.

President Erdogan has taken it a step further, recommending a "uniform" dress code for those on trial for alleged FETO offences.