BRUSSELS—Thousands of passengers' flights were canceled or were rerouted to other cities on Wednesday after a power cut knocked out the air-traffic-control system in Brussels.

An electrical failure caused the blackout Wednesday morning, said Dominique Dehaene, spokesman for Belgocontrol, the Belgian branch of Eurocontrol, which oversees air traffic in Europe. The power cut prevented flights taking off and landing at all five main Belgian airports, including the country's largest, Brussels Airport, just outside the capital.

Brussels Airport said on Twitter that 32 flights had been diverted to neighboring airports in Germany and the Netherlands and 147 had been canceled.

Airport authorities said some 20,000 people had been affected at that point. Those included the Greek negotiating team flying to Brussels for discussions on the country's bailout, Greek officials confirmed.

Flights crossing Belgium at above 24,500 feet were unaffected by the power cut because they are overseen from outside of Belgium, Belgocontrol said.

By late afternoon Belgian authorities were restoring the air-traffic-control system, allowing some flights to take off and land at Brussels Airport. Delays were expected throughout the day.

Subscribe to WSJ: http://online.wsj.com?mod=djnwires