BRUSSELS, Nov 21 - Belgium raised the alert status for its capital Brussels on Saturday to the highest level, warning the public to avoid crowds because of a "serious and imminent" threat of an attack, a government official said.

Brussels was placed on the top level of four in the government's threat scale after a meeting of top ministers, police and security services.

"The advice for the population is to avoid places where a lot of people come together like shopping centres, concerts, events or public transport stations wherever possible" a spokesman for the government's crisis centre said.

The level for the whole country was raised a week ago following the Paris attacks to level three out of four, implying a "possible or probable" threat. Previously, only certain sites, such as the U.S. embassy, were at level three.

Belgium, and its capital in particular, have been at the centre of investigations into the Paris attacks after it emerged that two of the suicide bombers had been living in the country. Three people detained in Brussels are facing terrorism charges.

The crisis centre spokesman declined to say what had led to the alert status change because investigations were proceeding.

"We cannot give more information... The work of federal prosecutors is still going on," he said.