Three people detained on Thursday in connection with an alleged plot to target Brussels on New Year’s Eve are to be held for a further 24 hours, Belgian prosecutors say.
Another three people who were also detained during the raids in and around the Belgian capital have been released.
Two other terror suspects were arrested earlier this week, and Brussels’ mayor cancelled New Year festivities.
French authorities have deployed more than 100,000 police to ensure security.
Paris saw its biggest public gathering since the deadly attacks there last month, but the city’s usual firework display did not take place.
Belgian authorities said the apparent plots to target New Year celebrations in Brussels are not related to the suspected network behind the Paris attacks.
The mayor of Brussels, Yvan Mayeur, said the decision to cancel the city’s New Year celebrations was based on intelligence suggesting they could be targeted.
The two men seized in Brussels on Sunday morning, named as 30-year-old Said Saouti and 27-year-old Mohammed Karay, both Belgian nationals, can be held for a month, a judge ruled on Thursday.
Both are members of a motorcycle club called the Kamikaze Riders. Some members have links to Islamist groups and to Belgians who have travelled to Syria to fight with so-called Islamic State, Belgian media report.
It was not clear whether the six taken in for questioning on Thursday morning were also connected to the motorcycle club.
Police seized material during Thursday’s raids including computers, phones and materials for playing airsoft – a military simulation game in which players fire replica weapons loaded with plastic pellets.