Six people were killed and eight wounded when gunmen opened fire at a Quebec City mosque during Sunday night prayers, in what Canadian Prime Minister Justin Trudeau called a "terrorist attack on Muslims".

Police said two suspects had been arrested, but gave no details about them or what prompted the attack, Reuters reports..

Initially, the mosque president said five people were killed and a witness said up to three gunmen had fired on about 40 people inside the Quebec City Islamic Cultural Centre. Police said only two people were involved in the attack.

"Six people are confirmed dead - they range in age from 35 to about 70," Quebec provincial police spokeswoman Christine Coulombe told reporters, adding eight people were wounded and 39 were unharmed.

The mosque's president, Mohamed Yangui, who was not inside when the shooting occurred, said he got frantic calls from people at evening prayers.

"Why is this happening here? This is barbaric,” he said.

Prime Minister Justin Trudeau said in a statement: "We condemn this terrorist attack on Muslims in a center of worship and refuge".

“Muslim-Canadians are an important part of our national fabric, and these senseless acts have no place in our communities, cities and country."

The shooting came on the weekend that Trudeau said Canada would welcome refugees, after U.S. President Donald Trump suspended the U.S. refugee program and temporarily barred citizens from seven Muslim-majority countries from entering the United States on national security grounds.

A Canadian federal Liberal legislator, Greg Fergus, tweeted: "This is an act of terrorism -- the result of years of demonizing Muslims. Words matter and hateful speeches have consequences!"

The premier of Quebec province, Philippe Couillard, said security would be increased at mosques in Quebec City and Montreal.

"We are with you. You are home," Couillard said, directing his comments at the province's Muslim community. "You are welcome in your home. We are all Quebecers. We must continue together to build an open welcoming and peaceful society".

New York Mayor Bill de Blasio said police were providing additional protection for mosques in that city following the Quebec shooting. "All New Yorkers should be vigilant. If you see something, say something," he tweeted.

News.Az