A prime suspect in Tuesday's Brussels bombings, Najim Laachraoui, was arrested on Wednesday in the city's Anderlecht district, several Belgian media said.
Police and prosecutors could not be reached for comment, but federal prosecutors announced they would hold a news conference at 1200 GMT, according Reuters news agency.
Police were hunting Laachraoui as being a man seen with suspected suicide bombers at Brussels airport.
A BBC report says, two of the other attackers have been named in Belgian media as the brothers Khalid and Brahim el-Bakraoui.
Twin explosions at the airport and another at a metro station on Tuesday left about 34 dead and 250 wounded.
Belgium is observing three days of national mourning.
So-called Islamic State (IS) has said it was behind the attacks and warned that more would follow.