Brussels, Jul 17 (IANS): Belgium has declared July 20 as a national day of mourning for the 21 people who died in the devastating flash floods.

Besides the victims, 18 persons were reported missing on Friday in the south and east of the country, reports Xinhua news agency.

"These are very exceptional circumstances. These are the worst floods our country has ever known," Prime Minister Alexander De Croo said at a press conference on Friday.

After three days of heavy showers and torrential rain, rivers and streams burst their banks, inundating entire streets and villages in some parts of the country.

The celebrations for July 21, Belgium's National Day, will still take place, but in a limited capacity.

Such bad weather "happens statistically once every 200 years. Normally, we find 100 millimeters of precipitation in July in the affected regions", said David Dehenauw, head of forecasting at the Royal Meteorological Institute of Belgium.

"That we are not able to reach the disaster victims is a lesson that we must learn from," said Walloonia's Minister President Elio Di Rupo on Friday morning on Belgium's RTBF radio station.

In the town of Verviers, near the city of Liege, disastrous floods submerged the city centre, upturning cars and damaging homes and shops along the high street.

Residents of all the municipalities along the Meuse river in Limburg, such as Maasmechelen, had to be evacuated after the river burst its banks and threatened to flood their homes.

King Philippe of Belgium and his wife visited the flood ravaged town of Pepinster near Liege on Friday, where around 10 houses collapsed.

The heavy downpours in the Belgian provinces of Luxembourg, Namur, Liege and Limbourg match what climate models predict for when the Earth warms up, suggesting direct links with global warming, the Belgian weekly Le Vive reported on Friday.

The European Union's Civil Protection Mechanism has been activated to tackle the heavy floods following a request for assistance from Belgium.