A midday shooting in the Canadian city of Montreal left a police officer, a civilian and the armed assailant dead on Monday.

The unidentified male suspect, who is believed to have acted alone, was shot and killed right away, the city's police chief said. One other police officer was severely injured and was reportedly in stable condition.

The attack unfolded in a diverse part of the city, Côte-des-Neiges. A witness saw a "gun sticking out of a window", heard shots and called police, investigators said.

Quebec's security minister said the motive was unclear.

After the attack, the Royal Canadian Mounted Police (RCMP) sent an alert to other police forces in Canada, the Globe and Mail newspaper reported. The bulletin is said to have warned about a document that had been circulating which allegedly encouraged citizens to shoot police officers.

Speaking to the AFP news agency, local resident Danny Wilk said he was near his home when he heard shots fired.

"I tried to take shelter in the nearby pizzeria, and that's when I saw the shooter, who looked ready to fire his weapon, dressed in military clothing," Wilk said.

The resident said he had seen the officer on the ground after being shot, before the attacker was himself shot by police.

Monday's incident marked the first time a Montreal police officer had been killed in the line of duty in nearly a quarter of a century.

Becoming emotional during a press conference, Montreal Police Chief Fady Dagher told reporters: "It's a nightmare."

The police department later named the officer who was killed as Mohamed Lamine Benredouane, 34. He had been part of the force since 2021.

French language public broadcaster Radio Canada said the gunman drew inspiration from the misogynistic "incel" movement - mostly online groups of young men who blame their lack of sexual activity on women.

One of Canada's most deadly mass killings, a 2018 vehicle-ramming in Toronto that killed 10, was connected to "incel" ideology.

The BBC has not confirmed the existence of any document relating to the Montreal attack.

The RCMP did not immediately respond to a request for comment.

Montreal police referred questions on the matter to the Bureau of Independent Investigations, which declined to comment on the investigation.

Ian Lafrenière, Quebec's domestic security minister, said earlier: "For now, we don't really know what the motive of this individual was."

The shooting began after 11:30 EDT (15:30 GMT) in Côte-des-Neiges, the most populated neighbourhood in Montreal.

Traffic in a nearby busy motorway was brought to a stop and local metro service was halted.

Just after 12:30, the province of Quebec issued an emergency alert warning residents of an armed suspect on the loose. The alert was lifted around 15:00.

Canada's Prime Minister Mark Carney said that he was "horrified" when he learned of the attack.

"My thoughts are with the victims, their loved ones, the first responders, and the entire community of Côte‑des‑Neiges," he wrote on social media.

Soraya Martinez Ferrada, the mayor of Montreal, said in a social media post: "My deepest condolences to the family, friends, and colleagues of the police officer who died in the line of duty."

Quebec Premier Christine Fréchette echoed those sentiments in a social media post adding that "such acts have no place here".