Schools and much of the Brussels metro system reopened on Wednesday as the Belgian capital started to return to normal after four days of lockdown, but troops on the street were a reminder it remains on the highest alert.

With police still hunting a local man who is a prime suspect in the 13 November attacks in Paris, along with up to 10 others authorities fear could be planning further violence, the government only plans to review the alert status next week.

The city's schools were protected by 300 additional officers, some of whom were standing guard outside school gates as parents dropped their children off.

Around half the city's metro stations were open, notably in the centre, with some 200 soldiers assigned to protect them.

Commercial and public life came almost to a halt over the weekend. Salah Abdeslam, 26, is on the run, suspected of being the eighth person Islamic State said took part in the attacks on Paris that killed 130 people. The Belgian government says he may be armed and dangerous and could be plotting further attacks.

More than two dozen people have been detained in Brussels since Nov. 13, though all have been released except five. They have been charged with terrorist offences, including three who drove Abdeslam in their cars after the attacks in Paris.

Concerts and sporting events have been cancelled but the weekend's Davis Cup tennis final between Belgium and Britain is due to go ahead as planned in Ghent, west of the capital.

In nearby Bruges, however, the mayor ordered that Thursday's Europa League soccer match between Club Bruges and the Italian side Napoli should be held in an empty stadium because police were too tied up on other duties to provide crowd control.

Concerns about how long the problems may last were also evident. A large New Year's party at an exhibition centre in Brussels has been cancelled, the event's organisers said.