Professor Pabhjot Singh was attacked last weekend.

A group of more than 20 young men viciously attacked a Columbia University professor over the weekend, apparently because they believed he was a Muslim.

Prabhjot Singh, 31, a medical doctor and an international relations professor at Columbia, had facial surgery to correct his jaw after the beating, which occurred near his home in Harlem, according to his friend.

Singh told the New York Daily News that one of the more than 20 men who attacked him while he walked in his Harlem neighborhood yelled, “Get him, Osama.”

Sikhs, who adhere to a monotheistic faith that originated in South Asia, are often mistaken for Muslims because they wear turbans and beards, and have been the victims of Muslim bias attacks. (A few days after the September 11 attacks, a Sikh gas-station worker was murdered by a man who was out for revenge against Muslims.)

A Sikh advocacy organization has counted more than 300 hate crimes against Sikhs like Singh in the years following the Sept. 11 attacks, including a 2012 attack by a white supremacist on a Sikh temple in Wisconsin that left six people dead.

“Frankly, they don’t know who we are, they don’t know we’re a distinct religion,” said Jasjit Singh, the executive director of the Sikh American Legal Defense and Education Fund. About half of Americans think Sikhism is a sect of Islam, the group’s research has found, and most Americans think of terrorism in association with the turban. “The figure they most commonly associate with a turban is Osama bin Laden,” he said.

Singh, the Columbia professor, was a leading advocate for the FBI to include Sikh Americans in its hate crime statistics, which the agency recently agreed to do. Attacks against Sikhs were formerly lumped together by the FBI with anti-Muslim crimes. But Singh argued that specific anti-Sikh bias also occurred, quoting portions of white supremacist websites that called for all Sikhs to “go home” to India.

“The government must begin tracking and counting anti-Sikh hate crimes, just as it must continue to vigorously combat bias and discrimination against all Americans, including Muslims,” Singh wrote in the New York Times last year.

Ironically, the professor may now be among the first to be included in the specific Sikh hate crime category he lobbied for, Jasjit Singh said. The NYPD's hate crime task force is looking into the incident.

“It’s incredibly sad,” Singh told the Daily News. “It’s not the neighborhood I know. I work in this community. It’s just not American.”