Belgium's federal prosecutor has said six people have been detained during house searches in Brussels in relation to an investigation into a plot to carry out an attack on the city on New Year's Eve.
A judge is set to decide on Thursday whether they can be held further.
On Wednesday, Brussels cancelled its New Year's festivities and fireworks display due to the threat of a terror attack.
The city's Mayor Yvan Mayeur said the decision was taken following a risk analysis by its crisis centre.
"In these circumstances, we can't check everyone," Mr Mayeur told the French-language RTBF network.
Two other people arrested earlier this week will be held for a month after being charged with threatening to carry out a terrorist attack, prosecutors said.