Belgium's interior minister, Jan Jambon, described the pre-dawn attack as a “cowardly act against our policemen.”
The victim was taken to hospital with non-life threatening injuries while the attacker was seriously hurt.
The country's justice minister, Koen Geens, said investigators were looking at all possible motivations, including links to extremist groups, but there were no clear indications yet.
Belgium has been under heightened security since the March 2016 attacks at Brussels' airport and subway, which left 32 people dead.