New York: Nationwide protests calling for further investigation into the shooting of 17-year-old Trayvon Martin a month ago have used a familiar garment as their rallying point: the hoodie.

National Basketball Association star LeBron James and his teammates on the Miami Heat as well as rap artist Sean Combs are among celebrities who've joined a social media campaign rallying "A Million Hoodies for Trayvon Martin".

More than 300,000 people have posted pictures of themselves in hooded sweatshirts to Twitter, Instagram and Facebook, according to organisers. Slogans on the photos include "We are all Trayvon" and "Hoodies Don't Kill, People with Guns Do." Martin, unarmed, black and wearing a hooded sweatshirt, was shot on February 26 in Sanford, Florida, by George Zimmerman, 28, a Neighbourhood Watch member who has claimed self-defence and hasn't been arrested.

Protesters have seized on Zimmerman's call with a 911 operator, in which he describes Martin's hoodie and dubs him "real suspicious", as racial profiling.

Viewing a person as dangerous because they're in a hooded sweatshirt is akin to being nervous about tinted windshields or dark sunglasses because they make it difficult to identify a person, said Lorraine Howes, a professor emerita at Rhode Island School of Design in Providence, where she teaches a course on the history of dress.

"You could put a sinister aspect on anything that conceals," Howes said. "It's not necessarily suspicious, it all depends on the aspect that has been put on it."

Hoodies trace their roots to monk robes in medieval Europe, according to fashion historians, and were introduced in the US in the 1930s by Hanesbrands's Champion brand for cold-weather work. The garment has become a modern staple of casual apparel, with most major brands putting their own spin on the hoodie, much as they have with denim or graphic T-shirts.

While Wal-Mart offers a women's Hanes fleece zip-up hoodie for $13 online, Juicy Couture sells a Malibu cashmere hoodie for $358 on its website. "It's now one of the more commonplace garments in the wardrobe of the world," said Daniel James Cole, an adjunct assistant professor of fashion history at the Fashion Institute of Technology.

"It's also typically used for branding — think of the many hoodies that have the name of the designer or company across the top. Just like jeans, there is a hierarchy expressed in how expensive the garment is."

Hooded sweatshirts gained popularity within hip-hop culture in the 1980s and 1990s; on his Ready to Die album in 1994 the late rapper Notorious B.I.G. wrote: "It don't make sense, goin' to heaven wit the goodie-goodies/Dressed in white, I like black Tims and black hoodies."

The garment's growing status prompted designers including Tommy Hilfiger to add hoodies to their high-end collections, reaching a larger audience, according to Howes. "I would date the huge surge to the time that Tommy Hilfiger became popular with both young white youths and black youths," she said.

"The thing that happened with Tommy Hilfiger is that instead of hip-hop clothing being very much in one segment of the population, it became across the culture."

Hoodies lost that urban association as they transitioned "into more fashionable circumstances and less marginalised ones", Cole said, as companies such as Gap started selling the garments. Facebook founder Mark Zuckerberg, 27, frequently sports a trademark outfit of a hoodie and jeans.