"It is a black day for our country, all Belgians share our immense sadness, there is no word to express what we feel," Di Rupo said in a statement.
"The government has decided a day of national mourning will be observed."
The statement followed a special cabinet meeting called after the accident by Di Rupo, who is to fly to Switzerland to the scene of the tragedy later in the day.
There were no immediate details on the day of mourning.
Di Rupo also said that ten Dutch, one German and one Polish child were among the 52 people aboard the bus when it crashed, six of them adults, including the two drivers.
Deputy premier and health minister Laurette Onkelinx said many of the children injured were in serious condition, including three in a coma, Belga news agency reported.
Parents of the children on their way home from a skiiing holiday when the coach slammed into a motorway tunnel on Tuesday night were also headed for Switzerland aboard a government plane.
A total of 28 people died in the crash near the town of Sierre, Swiss police said, including the six adults, but the exact cause was not yet known.