Three men and a girl of 16 were found with bomb-making materials by anti-terrorist police in a raid on a flat in the southern city.
The Belgian ambassador to France was on Thursday summoned to explain why the two arrested police officers had crossed the border with 13 migrants in their vehicle.
The detained were all from the region around Nice in south-east France, suspected of helping Mohamed Lahouaiej Bouhlel mount his attack on July 14th, when he drove a truck into Bastille Day crowds on the Nice seafront causing the deaths of 86 people.
The three Syrians, who police suspect were a 'sleeper cell', appear to have travelled via the same traffickers as three suicide bombers who blew themselves up near Paris in November 2015.
French interior minister Bernard Cazeneuve gave few details of the arrests but said three of those arrested were preparing terrorist attacks.
Three new arrests were made on Sunday as French investigators attempt to establish whether Mohamed Lahouaiej Bouhlel, the 31-year-old Tunisian who drove a heavy truck into Bastille Day crowds in Nice on Thursday, killing 84 people, received help from accomplices in preparing the massacre. Fresh evidence emerged this weekend suggesting he had carefully planned the attack, including CCTV footage of him reconnoitring the scene earlier last week. But despite a claim by the Islamic State group that Bouhlel was a "soldier" for the jihadist group, his motive remained unclear. Meanwhile, French health minister Marisol Touraine said on Sunday that “about 85 people” were still hospitalised after the carnage on July 14th, of which 18, including a child, were in a life-threatening condition. Graham Tearse reports.
The jihadist group claimed Tunisian who drove truck into crowd in Nice had followed its calls for such attacks; five people now in custody over the massacre.
Two England fans were arrested after clashes with local youths in the Old Port district of Marseille ahead of Saturday's England-Russia Euro 2016 match.
The UK-registered Audi was chased by Belgian police across border into France where its four occupents, reportedly Iraqis, were shot at and arrested.
Belgian prosecutors say Mohamed Abrini, who has admitted being third Brussels airport bomber, said the group changed target after arrest of Salah Abdeslam.
Belgian PM Charles Michel has confirmed three suspects were arrested in Friday's raids in Brussels, including Paris attacks fugitive Salah Abdeslam.
The three men and a woman, described by security services as radical Islamists, were arrested in Paris and a northern suburb in dawn raids.
Police arrested 14 people after they torched tyres and blocked bridges in the Channel port in protests against thousands of migrants gathered there.
Six migrants and two activists from No Borders group will stand trial Monday after they were arrested for storming a ferry in the Channel port.
Official said Salah Abdeslam, wanted for his part in the massacres, was not among the five arrested in and around Brussels on Sunday and Monday.