"What should have been beautiful summer days suddenly turned into dark and extremely sad days for our fellow citizens," Prime Minister Alexander De Croo told a news conference.
"These are exceptional circumstances that our country has not seen before", he added, Geo TV reported.
A week of rain finally came to an end after reaching levels in some places normally expected once in 200 years. But several communities across parts of Belgium were nervously watching as the river Meuse, which flows through the city of Liege in eastern Belgium, continued to rise and threatened to overflow.
In the eastern town of Verviers, the swollen river was still rushing through neighboring streets, where people gingerly tried to salvage ruined shops, homes, and cars.
Several towns and villages were submerged, including Pepinster near Liege, where around 10 houses collapsed. Belgium's king and queen visited the town on Friday, wading through flooded streets.
The government set next Tuesday as a day of mourning and decided to tone down festivities for Belgian National Day the day after.
Interior minister Annelies Verlinden said 20 people had lost their lives, with a further 20 missing.
RHM/PR