Citing unnamed police sources, local media including NBC News and The New York Daily News said the suspect was detained on Sunday night but has not been charged.

A police spokesman could not immediately confirm the reports.

Police have yet to establish a motive behind Saturday's murders and have said there was no evidence the men were targeted because of their faith.

But they have said nothing was being ruled out. Residents demanded authorities treat the brazen daylight shooting as a hate crime.

The gunman stalked the pair from behind and shot both in the head at close range at about 1:50pm (1750 GMT) in the Ozone Park neighbourhood of Queens, one of the city's five boroughs, police said in a statement.

The victims, identified as Imam Maulama Akonjee, 55, and Thara Uddin, 64, were wearing religious garb, police said.

Officers found them bleeding in the street and took them to a hospital where they were pronounced dead.

"While we do not yet know the motivation for the murders of Maulama Akonjee and Thara Uddin, we do know that our Muslim communities are in the perpetual crosshairs of bigotry," New York City Mayor Bill de Blasio said in a statement.

"Rest assured that our NYPD will bring this killer to justice."

The men were attacked a couple of blocks from the Al-Furqan Jame Mosque, from where they had just left afternoon prayers. Ozone Park, a diverse, largely working-class neighbourhood, is home to a growing number of Muslims of Bangladeshi descent.

Millat Uddin, 57, an Ozone Park resident not related to the one of the victims, said both men were born in Bangladesh. He said he was close to Akonjee, describing him as a "docile, calm" father of seven who was beloved in the neighbourhood.

"What matters most is harmless people have been shot dead, regardless of whether this was a hate crime," he said. "Our community's heart is broken."

Akonjee was carrying $1,000 in cash with him at the time of the attack but the money was not taken, the New York Times reported.

"I have never felt this kind of tension," said Nizam Uddin, 57, a taxi driver who said he knew both the cleric and his associate. He also was not related to the associate.

‘Makes all Muslims scared’

Police released a sketch of a male suspect with dark hair, a beard, glasses, and of medium complexion. He appeared to be in his 30s or 40s.

NBC reported the man being questioned matched the description.

The shooting appeared to be the most violent attack against local Muslim leaders in recent years, said Ibrahim Hooper, national communications director for the Council on American-Islamic Relations (CAIR), a civil rights and advocacy group.

A report in June by CAIR and the University of California at Berkeley said the number of recorded incidents in which mosques were targeted jumped to 78 in 2015, the most since the group began tracking them in 2009.

Hooper said he could recall incidents in which an imam was pushed, called names or otherwise harassed. "Things like that, but nothing of this nature, nothing where people were killed," he said.

CAIR offered a $10,000 reward for information leading to the arrest and conviction of the shooter.

Witnesses saw the assailant, dressed in a dark shirt and blue shorts, fleeing with a gun in his hand, police said. Surveillance footage showed the suspect following the victims.

Mohammed Ahmed, 22, who works at his father's corner store on Liberty Avenue just two blocks from the shooting, said he heard the shots while he was at work.

"It makes all the Muslims scared," he said. "Last time someone got shot in this neighbourhood that I know of was probably 2001."