Temperatures in Brussels are set to fall to as low as -10°C (14°F) in the coming week, with strong winds adding a further chill factor.
Vincent De Wolf, the mayor of Etterbeek, said he had asked police to take all homeless people to shelters even if they were unwilling to go.
“As a consequence, the mayor of Etterbeek has authorised, in case of absolute necessity, the detention of homeless people who are acting in a way that would endanger their safety by refusing shelter,” the Etterbeek council said in a statement.
Yvan Mayeur, the mayor of Brussels, said he had given similar orders to police and asked them to prioritise children sleeping outside.
Campaign groups estimate that there are more than 3,000 homeless people in the Brussels-Capital region.