The victims of a mass shooting at a Quebec City mosque were fathers, businessmen, a university professor and others who had gathered for evening prayers, a Muslim community leader said Monday as he recalled through tears the horror of the attack that killed six and injured 19 others.

"It's a very, very big tragedy for us," said Mohamed Labidi, the vice-president of the Centre Culturel Islamique de Quebec, the mosque where the attack happened Sunday night. "We have a sadness we cannot express."

Labidi said the victims were shot in the back.

"Security at our mosque was our major, major concern," he said. "But we were caught off guard."

The shooting took place just before 8 p.m. Sunday. Witnesses described chaos as worshippers scrambled to find friends and loved ones, while police responding to the scene called for backup.

One suspect was arrested shortly after the shooting.

Police said earlier they had arrested two suspects, whom court documents identified as Alexandre Bissonnette and Mohamed El Khadir. Police did not immediately clarify which man was the suspect and which one the witness.

Prime Minister Justin Trudeau and Quebec Premier Philippe Couillard moved swiftly to label the shooting a "terrorist attack."

Trudeau is planning to travel to Quebec City later Monday as the country reels from the deadly shooting.

In Quebec, Couillard said the attack had hit all Quebecers hard.

"All Quebecers are united in the solidarity we express today," he said. "Our society is a very open, tolerant and hospitable one, but we are not different from other societies, we have the same devils, xenophobia, racism...we should not be complacent in our society."

The mosque targeted in the attack said in a statement posted on its Facebook page late Sunday that it was grateful for the many messages of compassion it had received.

It is not the first time the mosque has been the target of threats. Last summer, a gift-wrapped pig's head was placed at the entrance to the building.

Authorities said the six men killed in the shooting were between the ages of 39 and 60. Of the 19 people injured — all men — two remained in critical condition on Monday, hospital officials said.

Majdi Dridi of the Muslim Association of Canada, said he knew two of the victims.

One was a work colleague who was a father of three little girls, he said.

"I don't know what to say, I just hope that his family and his children can have the patience to accept what happened," Dridi said.

Vigils are expected to be held in Montreal, Quebec City and Halifax later on Monday.

A number of Canadian Muslim groups have expressed shock and anger at the attack.

"Quebec Muslims are frightened right now," said Haroun Bouazzi, president of AMAL-Quebec, a Muslim human rights group based in Montreal. "We are urgently waiting for answers as to how and why such a tragedy could occur."

The National Council of Canadian Muslims called on law enforcement agencies around the country to increase security around mosques and Islamic centres.

"We are horrified by this despicable act of violence," the council's executive director Ihsaan Gardee said in a statement. "This act of wanton murder must be punished to the fullest extent of the law."

Gardee added that the council was "heartened" by the support expressed for the Muslim community by other Canadians.

The Ahmadiyya Muslim Jama also spoke out against the attack.

"We are deeply saddened by the attack that took place in a mosque in Quebec City and we pray for the rapid recovery of the wounded," said national president Lal Khan Malik. "We are taking steps to ensure that all members of the community feel secure and safe."

The mosque shooting also prompted world leaders to speak out.

U.S. President Donald Trump called Trudeau to express his condolences to Canadians and offered to provide assistance as needed.

French President Francois Hollande condemned what he called an "odious attack," while a spokesman for German Chancellor Angela Merkel called the shooting "despicable."

Pope Francis offered his condolences and called for mutual respect among people of different faiths.

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