The Latest on the security situation in Belgium and arrests made (all times local):

3:30 p.m.

Belgian authorities have detained six people in connection with last year's foiled attack on a Thalys express train to France.

The Federal Prosecutor's Office says six houses in the greater Brussels area were also searched in the Monday operation.

It says six people have been taken in for questioning and that an investigating judge would decide later Monday whether they should be kept in detention.

No arms or explosives were found.

The prosecutor's office says no further information will be made public about those detained or items seized in the searches.

___

9:30 a.m.

Brussels remains on high alert with increased security after a weekend sweep which left three in jail facing terrorism charges.

The MIVB metro system says a half dozen subway stations have reduced entry on request from authorities but all subway lines were running during the morning rush hour Monday.

On Saturday, authorities charged three men with terror-related crimes after weekend raids and the detention of 40 people in a major investigation. Authorities said the probe required "immediate intervention" because they feared a new attack was close.

Prime Minister Charles Michel said the nation would remain "extremely vigilant, hour by hour," and that the terror level across the country would remain at the second-highest level, meaning a threat of an attack "is possible and likely."