Brussels terror threat: Special forces arrest four people after warnings of 'imminent threat'

Brussels has raised its terrorism alert to its highest level, suspending its underground and deploying armed security forces to patrol the city.

The government's crisis centre raised the alert to 'Level Four', indicating a "serious and immediate threat", amid a reports that police had found a cache of weapons and ammunition during a raid on an apartment in Molenbeek, the poor suburb of Brussels that was home to the suspected mastermind of the Paris attacks that killed 130 people.

The Belgian Federal Prosecutor's office later confirmed the discovery in the home of one of three people arrested in Belgium in connection with the Paris attacks.

In a written statement the office said no explosives or suicide bomb belt had been found in the search. The Belgian paper Dernière Heure reported that chemicals had also been found in a raid in Molenbeek, but this was not confirmed by the prosecutor.

Special forces arrested four people at around 3pm on the Place du Grand Sablon, Belgium media reported.

Earlier, Belgium's Prime Minister Charles Michel said the terror alert had been raised "based on quite precise information about the risk of an attack like the one that happened in Paris".

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He told a news conference on Saturday morning that it was feared "several individuals with arms and explosives could launch an attack ... perhaps even in several places".

"We urge the public not to give in to panic, to stay calm. We have taken the measures that are necessary," he said.

Jan Jambo, Belgium's interior minister, said the country's situation was "serious" but under control.

Around the capital, heavily armed police and soldiers were on patrol at key intersections of the Belgian capital, home to more than one million people and the location for key EU and Nato offices.

The metro system is likely to remain shut until Sunday afternoon when the authorities will review the terror alert.

Shopping centres were also closed.

Residents have been advised to avoid areas where large numbers of people gather, including concerts, major events, train stations and airports, public transit and commercial districts.

By the afternoon the city's main streets appeared deserted.

The government recommended the cancellation of professional soccer games scheduled over the weekend. It also recommended that the 19 communes that make up the Brussels region consider cancelling all other major events due to take place.

A rock concert by the French musician Johnny Hallyday, dues to take place in Brussels, has also been cancelled.

A string of terrorist attacks in recent years have had links to Belgian capital's Molenbeek neighbourhood, just 25-minutes walk from the centre of Brussels.

Meanwhile, a manhunt is under way for Salah Abdeslam, a suspected attacker who escaped and is thought to be in Belgium. French police stopped Abdeslam at the Belgian border the morning after the attacks but then let him go.

ABC News reported the fugitive Abdeslam had been in touch with friends via Skype and mobile phone who had urged him to turn himself in.

(Additional reporting by agencies)