A bus driver rammed his vehicle into a day care center near Montreal Wednesday, killing two four-year-old children in what police believe was a deliberate act.

One child died on the scene and another in an ambulance, Canadian authorities said, after several children were trapped under the vehicle when it slammed into the building in the suburb of Laval at around 8:30 am (1330 GMT).

"We think that this is deliberate, but we don't know the motive," local police chief Pierre Brochet told AFP.

The 51-year-old suspect, named as Pierre Ny St-Amand, seemed agitated as he appeared via video link in court later Wednesday. He faces nine counts including premeditated murder and attempted murder.

Six other children transported to hospital are out of danger, said Laval police spokeswoman Erika Landry.

Hamdi Ben Chaabane, who lives nearby, said he arrived at the scene to find the driver acting erratically.

"He opened the door. He took off all his clothes. He was totally naked," Chaabane said, according to broadcaster CBC. "We don't know why he did that. We dove on him. We tried to subdue him," he said, describing the man as "in another world."

St-Amand refused to speak but nodded to answer the judge's questions during his court appearance, smiling broadly and repeatedly trying to sit up from the bed to which he was handcuffed, an AFP journalist in the room observed.

The case will resume on February 17, crown prosecutor Karine Dalphond said, with St-Amand remaining in a psychiatric hospital in the meantime.

The bus he drove appeared to have smashed into a side of the building which is near a parking lot away from the nearest road, remaining there late Wednesday afternoon with part of the roof collapsed on its front portion.

Another parent at the scene, holding her own child, told Radio Canada it was the center's class of oldest children that was hit.

"I know that there are no words for parents who have lost their children this way," Prime Minister Justin Trudeau told reporters.

"All Canadians are grieving with them, and we'll do whatever we can to support them in their horrific pain in the coming days, months and years of grief," he said.

No adults were hurt in the collision.

- 'Terrible tragedy' -

Some of the victims were taken to Sainte-Justine hospital.

"There is nothing more cruel than attacking our children," Quebec Premier Francois Legault tweeted, praising emergency workers who responded to the scene.

"I keep thinking about the families and loved ones of the children who died, as well as those who were injured," he said.

Legault said he and opposition party leaders planned to go back to Laval Thursday "to offer my support to the families and to the personnel affected by this terrible tragedy."

Day care worker Mona Gilot, 54, told AFP she was angry.

"My head is spinning. It's really difficult for me to speak," she said.

After the crash, parents rushed up the snow-covered sidewalk to collect their children from the center, which usually has about 80 or 85 children attending, while a nearby school was transformed into a crisis headquarters.

"I am shocked by this news. The investigation continues to understand this tragic series of events," tweeted Stephane Boyer, the mayor of Laval.

"All of my support to the parents at these difficult moments," he added.

"Laval is wholeheartedly with you and will be there for you."

Quebec flags will be lowered and the Montreal Tower at the city's Olympic stadium will be dimmed Wednesday evening as will the emblematic CN tower in Toronto, in honor of the victims.

In 2018, a Canadian man rammed a rental van into a crowd of pedestrians in downtown Toronto, killing 11 and injuring 16 in the deadliest such attack in the country's history.