Police were investigating a single suspect in a shooting at a Quebec City mosque that killed six people, with a second person who was arrested now considered a witness, authorities said on Monday.

The sole suspect in the attack on Sunday evening prayers was Alexandre Bissonnette, a French-Canadian university student, according to a source familiar with the matter.

The man now considered a witness was of Moroccan descent although his nationality was not immediately known, the source said.

Police declined to give details of those arrested or possible motives for the shooting at the mosque, the Centre Culturel Islamique de Québec.

Authorities initially said they had arrested two suspects, but in a Twitter message, the Royal Canadian Mounted Police said that "following the investigation, the second individual is now considered as a witness".

Police said on Monday morning they were confident no other suspects were involved in the attack.

"They consider this a lone wolf situation," the source said.

In addition to the six killed, five people were critically injured and 12 were treated for minor injuries, a spokeswoman for the Quebec City University Hospital said.

Canadian Prime Minister Justin Trudeau earlier called the shooting "a terrorist attack on Muslims". He was heading to Quebec City later on Monday, a spokesman said.

US President Donald Trump called Trudeau to express his condolences "and offered to provide any assistance as needed", said Trudeau spokesman Cameron Ahmad. He gave no further details about of the call.

The shooting came over a weekend when Trudeau said Canada would welcome refugees, speaking in response to Trump's order to halt the US refugee programme and to temporarily bar citizens from seven Muslim-majority countries from entering the United States.

Trump's action, which the president said was aimed at protecting Americans from the threat of attacks by militants, was widely condemned in the United States and abroad as targeting Muslims.

Father of four among those killed

A father of four, the owner of a meat shop near the mosque, was among those killed, said Pamela Sakinah El-hayet, a friend of one of the people at the mosque.

The mosque concierge was killed, as was Ahmed Youness, a 21-year-old student, El-hayet told Reuters. One of El-hayet’s friends, Youness’ roommate, was in the mosque at the time of the shooting. He was unharmed, she said, but in total shock.

Ali Assafiri, a student at Université Laval, said he had been running late for the evening prayers at the mosque, near the university in the Quebec City area. When he arrived, the mosque had been transformed by police into a crime scene.

"Everyone was in shock," Assafiri said by phone. "It was chaos."

Université Laval is the oldest French-language university in North America, with 42,500 students.

Vigils were planned for Montreal and Quebec City, the provincial capital, as well as in Edmonton later on Monday. There was an outpouring of support for the mosque on social media.

Citizens for Public Justice, a group of Canadian Christians, churches and other religious congregations, expressed their solidarity with the Muslim community of Quebec City.

"Last night's shooting, targeting people of faith during their worship and prayer, is a deplorable attack on all Canadians and our most deeply-held values," the group's executive director, Joe Gunn, said.

While the motive for the shooting was not known, incidents of Islamophobia have increased in Quebec in recent years.

The face-covering, or niqab, became a big issue in the 2015 Canadian federal election, especially in Quebec, where the majority of the population supported a ban on it at citizenship ceremonies.

Pope Francis offered his condolences to Cardinal Gerald Cyprien LaCroix, Archbishop of Quebec, who was visiting Rome on Monday.

"The pope underlined how important it is in these moments that everyone remains united in prayer, Christians and Muslims," the Vatican said in a statement.