The ruling prompted the public, already outraged by reports of the crimes and the slowness of investigators to respond, to take to the streets. The dismissal also followed press reports that Mr. Connerotte was on the verge of disclosing the names of senior government officials who had been recognized on confiscated videotapes, and during allegations of a high-level cover-up.

The march had been planned before Mr. Connerotte's dismissal last week, and gained momentum in the days following, which were marked by spontaneous protests around the country aimed increasingly at the nation's entire political and judicial class.

People from all ages and parts of the country gathered in the capital today, some wearing the white clothes and caps urged by organizers as a demonstration of hope.

Some 1,400 police officers were assigned to control the crowd, though the protesters marched peacefully, heeding calls for calm from King Albert, Prime Minister Jean-Luc Dehaene and the parents of the missing children.

''I am here to show solidarity with the parents,'' said one marcher, Gerda Zammattio, the mother of a three-year-old daughter, Naomi. ''What has happened in the last few weeks moved me greatly. The system must become more humane.''