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.
Separately, Belgian police arrested a 10th suspect over the 13 November attacks in Paris, in which 130 people were killed.
The Belgian national, identified as Ayoub B, was detained on Wednesday during a raid in the Brussels district of Molenbeek. He has been charged with terrorist murder and participation in the activities of a terrorist group.
Security was stepped up in other major European cities ahead of New Year celebrations, including Paris, London, Berlin and Moscow, with officials wary of possible terror plots. Munich police issued a warning of an imminent threat there.
Earlier this week, Austrian police claimed a "friendly intelligence service" had tipped them off that major European capitals were at risk of being attacked over the holiday period.
While the French capital cancelled its main New Year fireworks display, the traditional gathering on the Champs-Elysees took place amid tight security.
Arrests in Ankara
On Wednesday, Turkish police arrested two suspected IS members over an alleged plot to attack celebrations in Ankara.
State media said they were planning two separate attacks on crowded areas. Suicide vests and explosives were found during police raids.
Security is also being tightened in cities where the authorities say there is no specific intelligence about a possible attack.
Berlin bag ban
In Berlin, backpacks and fireworks were prohibited and bags searched on the "party mile" leading up to the Brandenburg Gate.
Up to a million people were believed to have attended the celebrations.
Berlin's interior minister Frank Henkel encouraged party-goers to not allow fear to sour their celebratory mood. "Caution is a good counsellor, fear is not," he told broadcaster RBB.
Precautions in London
London's Metropolitan Police have deployed 3,000 officers in the inner city, including extra armed officers.
More than 100,000 people are expected to watch the Mayor of London's fireworks show, a ticketed event.
"Our plans are purely precautionary and not as a result of any specific intelligence," said a spokesperson.
Australian officials had assured revellers that thousands of extra police would patrol major cities, but urged residents to celebrate. "Don't change your way of life," said Melbourne's Lord Mayor Robert Doyle.
The US embassy in Dhaka, Bangladesh, warned its citizens of "possible attacks" against hotels and clubs in the city in connection with New Year's Eve celebrations.
And a number of other cities around the world have been put on high alert or called up additional police officers, including Tokyo, Kuala Lumpur and New Delhi, where police and anti-terror squads conducted mock terror drills this week.