A Belgian anti-terrorism judge has charged a suspect with involvement in the Paris attacks, the federal prosecutor's office said this evening, the fourth person to face charges in Belgium over the attacks in Paris on 13 November.

The suspect was arrested in a series of police raids in Belgium last night along with 15 other people who were all released without charge, as authorities warned of a Paris-style plot facing Brussels.

"The investigating judge specialised in terrorism cases placed into custody a man arrested during the operations of last night.

“He is charged with participating in activities of a terrorist group and with terrorist attack [Paris]," the prosecutor's office said in a statement.

"After thorough interrogation by the federal judicial police, the remaining 15 persons were released by the investigating judge."

Mohammed Amri, 27, and Hamza Attou, 20, were charged last Monday on suspicion of helping alleged Paris attacker Salah Abdeslam with escaping to Brussels after the 13 November carnage in which 130 people died.

A third person who has not been publicly named was also charged with involvement in the Paris attacks on Friday and is reported to have helped Salah when he was dropped off in the Belgian capital.

One of the main suspects behind the Paris attacks, Salah Abdeslam, who is from Brussels and who has been on the run since he left the French capital hours after his elder brother blew himself up at a café there, continues to elude a Europe-wide manhunt.

A third brother, who was not involved, said Abdeslam may have thought better of going through with the killing.

Belgian police fear he may have returned home to launch new attacks.

"What we fear is an attack similar to the one in Paris, with several individuals who could possibly launch several attacks at the same time in multiple locations," Prime Minister Charles Michel told a news conference yesterday evening.

Abdeslam travelled through Italy in August with a companion, but his presence caused no alarm because he was not a wanted man at the time, an investigative source said this afternoon.

He boarded a ferry in the southeastern Italian port of Bari on 1 August en route to Patras, Greece, the source said, with another man.

Abdeslam returned from Patras to Bari by ferry on 5 August. The source gave no further details.

Schools and Metro to reopen Wednesday

Brussels will remain under the highest level of alert for another week due to an ongoing terrorism threat, but schools and the underground train system will reopen from Wednesday, Belgian Prime Minister Charles Michel said this evening.

"The crisis centre decided to maintain the alert level four, which means the threat remains serious and imminent," Michel told a press conference, adding the threat level will be reviewed again next Monday.

"We want to thank the people for their calm and understanding," he added.

The army and police will continue to be deployed in force and the country will reduce the number of events with large crowds, for fear of a repeat of the Paris gun and suicide bomb attacks, Mr Michel said.

But he added his government was trying to bring the country "back to normal as quickly as possible" while working with the security services.

It decided to reopen schools and the underground metro from Wednesday.

"For schools, that means that in the coming hours, we will guarantee a level of security everywhere on the country's territory," the prime minister said.

"As for the metro, the aim is to reopen the metro gradually, but starting on Wednesday."

The rest of the country will remain on alert level three, which means an attack is considered possible and the threat credible.

Brussels has been locked down since Saturday with armed police and troops patrolling quiet streets.