"This is a tragic day for all of Belgium," Prime Minister Elio Di Rupo said in a statement before flying to the scene of the disaster.
"Words are useless," he told national radio and TV networks providing round-the-clock coverage of the horrific crash. "We are speechless."
Di Rupo flew to Switzerland as parents of the children returning from a skiing holiday gathered at one of their schools before also heading for Switzerland aboard a government plane.
At Heverlee, near Leuven, home to some of the other crash victims, the atmosphere was fraught as it was not yet known who had died and who had survived.
Relatives gathered at the Sint-Lambertus School while students were ferried to another school.
"We know some of the children are OK, but we don't have names," said the headnaster of Sint-Lambertus, some 30 kilometres east of Brussels, where 24 children, a teacher and a teacher's assistant were registered.
"We asked the parents to come and we are trying to comfort them," Marc Carels told RTL television.
Belgium Archbishop Andre-Joseph Leonard, visibly moved, visited the Catholic school and said he was praying for the families. "The time will come later to find the words," he said.
The European Parliament in Strasbourg, France, was to hold a minute of silence at noon.
A total of 28 people died in the crash in a motorway tunnel on Tuesday night, Swiss police said, including the two drivers.
The Belgian government said it was making arrangements to have relatives of the victims flown out and accompanied to Switzerland.
The bus, which was carrying 52 passengers, suddenly swerved to the right and smashed into the concrete wall of an emergency lay-by. Another 24 children were reported injured in the crash.
Foreign Minister Didier Reynders said two army aircraft would be used to fly the relatives to Switzerland but officials later said they would leave in a single Airbus.
"The aim is to accompany the families who want to go to Switzerland," said Reynders, who was speaking from Vietnam where he is on an official visit.
A psychological support team was also on hand, he added.
"Our first thought was the distress of the families," he said.
Peter Vanvelthoven, the mayor of Lommel in northeast Belgium, where some of the schoolchildren went to school, said they were also trying to help the families.
"We have arranged a reception at the school, first for the parents, for the children and for the teachers, too," he said.
"I'm at a loss for words," Transport Minister Melchior Wathelet told RTBF radio. "Terribly hurt, terribly moved.
"We are all thinking like parents, with this terrible thought for the those parents who will not see their children coming back today," he added.
"Yesterday evening, they were looking forward to seeing them and they won't see them again."
The Belgian transport company that ran the coach that crashed was Toptours, based in Aarschot, central Belgium, said Wathelet.
"The company ... enjoys an excellent reputation," he added.
"It has always respected the rules," regarding safety, he added.
The two coach drivers who died in the accident had arrived in Switzerland the day before. The coach had been built in 2002.