BRUSSELS — The Belgian authorities imprisoned Patrice Lumumba as a rabble-rouser. When, shortly afterward, he became the first prime minister of an independent Congo, once their largest and most profitable colony, they supported a military coup that culminated in his killing.

On Saturday, they named a square in Brussels after him.

The former Square du Bastion — which lies near the city’s 14th-century walls, and leads to the Matonge neighborhood, home to a large Congolese diaspora population — became Square Patrice Lumumba 58 years to the day after Mr. Lumumba declared his country’s independence.

The cheering crowds at the ceremony on Saturday, however, masked Belgium’s complex relationship with its former colony. Belgium’s rule over what is now the Democratic Republic of Congo is a sensitive topic here. Most Belgians have at least one ancestor who played some role in colonial-era Congo, and Belgium has officially admitted involvement in Mr. Lumumba’s death.

“By inaugurating this square, we’re not repairing the past, we’re not closing a chapter of history,” said Philippe Close, the mayor of Brussels. “Today, by inaugurating this square, we forget nothing.