The speed restrictions are in force on several motorways and orbital ring roads.
The poor air quality triggered the smog alert. In several spots particulate matter concentrations exceeded the norm of 70 micrograms per cubic metre.
At a string of measuring points concentrations of 100 micrograms per cubic metre were measured.
Pollution is trapped in Belgium because there is hardly any wind.
Hajo Beeckman of the Flemish Traffic Centre says that the smog alert is in force for 48 hours: "Monday or Tuesday a decision will have to be taken on whether to extend the alert or end the traffic restrictions".
On Sunday night smog alert signs were made visible at places where speed restrictions are in place.
The signs occur every 2 kilometres. There are also signs at motorway access points. Electronic signs overhead are also used to alert motorists.
Hajo Beeckman says: "There won't be extra police checks on the motorways, but all the police traffic operations planned for today will focus on the speed restrictions introduced by the smog alert. Speeding drivers face being caught. Unmanned speed cameras have been adjusted to take account of the 90 km/h limit."
Mr Beeckman added: "90 km/h should lead to calmer traffic flows, fewer jams and less chance of an accident".
Where are the restrictions?
The smog alert applies on most of the motorways between Leuven, Brussels, Ghent and Antwerp and on orbitals around these cities.
There are also restrictions between Antwerp and Hasselt and in the Kortrijk area of West Flanders.
