New York City police have released a sketch of the suspect they say fatally shot the leader of a mosque and a friend as they left afternoon prayers.

Witnesses described the shooter of 55-year-old Imam Maulama Akonjee and 64-year-old Thara Uddin as a man with a medium complexion, last seen wearing a dark shirt and blue shorts.

Police released a sketch of a dark-haired, bearded man wearing glasses.

The gunman approached his victims from behind as they left the Al-Furqan Jame Masjid mosque in Queens shortly before 2pm on Saturday.

Video surveillance showed the suspect then fled south on 79th Street with the gun still in his hand.

Police have not determined a motive, and said there was no indication the men were targeted because of their faith.

But members of the Bangladeshi Muslim community served by the mosque worry it could be a hate crime, and more than 100 people attended a rally on Saturday night.

Naima Akonjee, the imam's daughter, says her father did not have "any problems with anyone."

The Anti-Defamation League is expressing solidarity with New York City's Muslim community in the wake of the shootings.

The organisation said that while the motive for the crime is still unknown, nothing can justify the killing of two men walking from their place of worship.

It is urging the NYPD to investigate the shootings as a possible bias crime.

The ADL fights anti-Semitism around the world through programs and services.

More than 100 people attended a rally on Saturday night and chanted "We want justice!"

The Council on American-Islamic Relations held a news conference near the shooting scene, where Kobir Chowdhury, a leader at another local mosque, said: "Read my lips: This is a hate crime" directed at Islam.

"We are peace-loving."

Sarah Sayeed, a member of Mayor Bill de Blasio's staff who liaises with Muslim communities, attended the rally.

"I understand the fear because I feel it myself," she said.

"I understand the anger. But it's very important to mount a thorough investigation."

Letitia James, the city's public advocate who serves as a watchdog over city agencies, said: "This violence is as alarming as it is senseless."

She urged the police department to "vigorously" investigate the killings.

Members of the community had felt animosity lately, with people cursing while passing the mosque, said worshipper Shahin Chowdhury.

He said he had advised people to be careful walking around, especially when in traditional clothing.

He called the imam a "wonderful person" with a voice that made his Koran readings especially compelling.

Worshipper Millat Uddin said Mr Akonjee led the mosque for about two years and was a very pious man.

"The community's heart is totally broken," said Mr Uddin, who is not related to Thara Uddin.

"It's a great misery. It's a great loss to the community and it's a great loss to the society."

Naima Akonjee, 28, one of the imam's seven children, said she rushed to her parents' home after the shooting.

She said her father was a caring man who would call her just to check up on whether she had eaten properly.

Neighbours also described Mr Uddin as a pious and thoughtful man who prayed five times a day and went to the mosque. While at home, they said he would water his garden and one next door.

"A very honest, wise man ... (And) a very helpful guy," said neighbour Mohammed Uddin, who is also not a relation of Thara Uddin's.