Police in Belgium are trying to determine what motivated a man to open fire in a busy marketplace in Liège, killing at least six people.

Nordine Amrani, 33, who allegedly threw hand grenades and emptied a rifle onto a crowded market square, was one among the six people who have been reported dead. Belgium's Home Affairs Minister Joelle Milquet said four people remained in a critical condition.

The city is to hold a minute's silence and a vigil at midday on Wednesday. Liège's mayor Willy Demeyer said the attack had sown sorrow in the heart of the city.

Prime Minister Elio Di Rupo said it was an isolated case. The whole country shares the pain of the families affected. We share the shock of the population, he said as he visited the square. This is an isolated case. This is not about terrorism. 

Amrani, a resident of Liège, had been jailed for 58 months in September 2008 for possessing firearms and drugs. Amrani is reported to have been on parole and on Tuesday was asked to go to the police station for an interview in connection with the charges against him.

At no moment in any of the judicial proceedings against him was there a sign of unbalance, Daniel Reynders, the public prosecutor for Liège, told reporters.

Officials said Amrani left his home with a backpack, armed with hand grenades, a revolver and an FAL assault rifle. He walked alone to the busy Place Saint-Lambert, the central entry point to downtown shopping streets, and climbed onto an overpass that gave him a view over the square, which was bedecked with a huge Christmas tree and crowded with shoppers.

At around midday, he threw three grenades at people waiting for buses then opened fire, sending hundreds of people fleeing in panic. Officials said shot himself in the head after that.

Belgium's King Albert II and Queen Paola also visited Place Saint-Lambert on Tuesday evening to pay their respects.