King Albert II and thousands of mourners on Wednesday remembered the 28 victims of last week's bus crash in a Swiss tunnel during a memorial service centering on the 22 schoolchildren whose promise of youth was shattered by sudden death.

Under a sparkling sky in northern Lommel, soldiers took part in a solemn procession that carried 15 coffins into a 5,000-capacity hall. The brown casket contained the remains of a teacher, the 14 white ones held the bodies of children who were on the cusp of their teenage years.

The students and the teacher were from one of two schools in northern Belgium that shared a bus for a traditional "snow class" vacation in Switzerland. They were returning from that exuberant holiday on March 13 when tragedy struck. Their bus, carrying 52 people, slammed into a tunnel wall. In addition to the dead, 24 children were injured.

The cause of the crash remains under investigation.

In Leuven, close to Brussels, a vigil with torches and candles lit up an evening sky as hundreds of students and citizens attended to show their respect for the victims of the second school which was hit by the tragedy. A full memorial service is set for early Thursday.

"Is there something worse than parents who lose what they love most?" asked Lommel Mayor Peter Vanvelthoven at the opening of Wednesday's service.

In a gloomy three-hour ceremony in the blackened arena, even a bittersweet attempt by pupils to briefly lift spirits with the up-tempo evergreen song about "Cheerful Friends" failed to break the leaden atmosphere.

Fathers and mothers, brothers and sisters of the dead gave speeches reminiscing about their lost loved ones, including a daughter's favorite meal and a son's newly decorated bedroom.

"The snow classes were a true feast. Happy, smiling kids. Excellent weather, great snow," ski monitor supervisor Marina Claes said of the days preceding the tragedy. She spoke of snow barbecues, surprise parties, and on the last day "everyone got their much desired ski diploma," she said.

Then the kids hopped on the bus home.

Six of the victims were Dutch nationals, and Belgium's King Albert II was joined at the service by Dutch Crown Prince Willem Alexander, Belgian Prime Minister Elio Di Rupo and his Dutch counterpart, Mark Rutte.

The 15 coffins were lined up at the front of the hall for the hour-long ritual. The dignitaries walked over to the grieving families to try to give them some measure of comfort.

The "snow class" is a rite of passage from childhood to teenage years for countless Belgian children, and the contrast between the happiness of the trip and the horror of the crash has moved virtually everyone in this nation of 11 million.

At the service, families pinned red roses into the center of a giant heart of yellow roses. As the famed Scala choir sang a soulful rendition of U2's "With or Without You," the coffins were carried outside into the bright sunlight and handed back to the care of families for funeral arrangements.

About 5,000 people attended the ceremony at the Soeverein Arena, and several thousand others followed the service on giant screens outside.

"It hurts to come all the way here, it hurts, it hurts to watch these 14 coffins in front of us," said Filomena Matongo, who had traveled 115 kilometers (75 miles) from Brussels to Lommel. "As I am a parent, it really moved me."

Three girls who were on the trip remain in Switzerland. They were badly injured, but they have regained consciousness and are out of immediate danger.

The crash near the Alpine town of Sierre was one of the worst road disasters in Swiss history.