"The crisis centre decided to maintain the alert level four, which means the threat remains serious and imminent," Michel told a press conference, adding the threat level will be reviewed again next Monday.
"We want to thank the people for their calm and understanding," he added.
The army and police will continue to be deployed in force and the country will reduce the number of events with large crowds, for fear of a repeat of the Paris gun and suicide bomb attacks on November 13, Michel said.
It decided to reopen schools and the underground metro from Wednesday.
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"For schools, that means that in the coming hours, we will guarantee a level of security everywhere on the country's territory," the prime minister said. "As for the metro, the aim is to reopen the metro gradually, but starting on Wednesday."
The rest of the country will remain on alert level three, which means an attack is considered possible and the threat credible.
Brussels has been locked down since Saturday with armed police and troops patrolling quiet streets.