The office of Canadian Prime Minister Justin Trudeau has forced US outlet Fox News to retract a "misleading" and "irresponsible" tweet that falsely described the suspect of the Quebec mosque attack as being of "Moroccan origin".

Six people were killed in the shooting, and although authorities initially said they had arrested two suspects, they later said the second individual — of Moroccan descent — was now "considered as a witness".

The first, Alexandre Bissonnette, 27, was charged with six counts of murder and five of manslaughter over the attack, which occurred during evening prayers and left 17 people wounded.

Fox News posted the tweet — which said "Suspect in Quebec mosque terror attack was of Moroccan origin, reports show" — after the police made the clarification.

The tweet made no mention of Bissonnette, a French-Canadian, and although the article it linked to was later updated, the tweet continued to circulate.

Kate Purchase, director of communications for Mr Trudeau, said on Twitter that she had sent an email to Fox News, asking them to either retract or update the tweet.

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"Sadly, this misleading information has been left to stand on the Fox News Channel's twitter account and continued to circulate online even now," she said.

"Canada is an open, welcoming country that stands by its citizens … we are stronger not in spite of our differences, but because of them.

"These tweets by Fox News dishonour the memory of the six victims and their families by spreading misinformation, playing identity politics, and perpetuating fear and division within our communities.

"If we allow individuals and organizations to succeed by scaring people, we do not actually end up any safer. Fear does not make us safer. It makes us weaker. Ramping up fear and closing our borders is not a solution."

Fox News later deleted the tweet: "FoxNews.com initially corrected the misreported information with a tweet and an update to the story on Monday. The earlier tweets have now been deleted," said managing director Refet Kaplan in a statement.

"We regret the error."

"Thank you Fox News for deleting the tweet. We appreciate it," Ms Purchase tweeted in response.

'We will stand with you'

Among the six men killed in the shooting were a butcher, a university professor, a pharmacist and an accountant, according to police and Canadian media.

Large crowds gathered at Parliament Hill and other locations across the country for candlelit vigils, while the lights of the Eiffel Tower in France were dimmed for one minute in honour of the victims.

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

Student Ali Assafiri said he had been running late for the evening prayers at the mosque. When he arrived, the mosque had been transformed by police into a crime scene.

"Everyone was in shock," Mr Assafiri said.

"It was chaos."

Large crowds assembled in cities across Canada. ( AP: Ryan Remiorz )

Mr Trudeau told the House of Commons in Ottawa: "Make no mistake, this was a terrorist attack," adding a personal message to Canada's 1 million Muslims.

"Know that we value you. You enrich our shared country in immeasurable ways. It is your home. Last night's horrible crime against the Muslim community was an act of terror committed against Canada and against all Canadians," he said.

"We will grieve with you. We will defend you. We will love you. And we will stand with you."

The attack was out of character for Quebec City, a city of just over 500,000 which reported just two murders in all of 2015. Mass shootings are rare in Canada, where gun control laws are stricter than in the United States.

Sorry, this video has expired A witness initially said up to three gunmen fired on about 40 people inside the mosque.

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