The arrests came shortly before many parties began across Belgium on Saturday to watch live broadcasts of the country’s Euro 2016 group stage match against Ireland and there were reports such events could have been the targets.

The prime minister, Charles Michel, said the nation would remain “extremely vigilant, hour by hour”, but that the terror level would remain at the second-highest, meaning a threat of an attack “is possible and likely”.

Belgians have been living under this level since the March attacks on the Brussels subway and airport which killed 32.

“It will be the case in the coming hours that we will take additional and adapted measures,” Michel said after a meeting of the nation’s security council. He refused to elaborate on the nature of the threat.

The federal prosecutor’s office said on Saturday that homes and car ports were searched in 16 municipalities, mostly in and around Brussels. The statement said there were no major incidents during the raids and no arms or explosives were found.

Forty people were taken for interrogation, with 12 of them being arrested. A judge was due to rule on their continued detention later on Saturday.

“It is not over. We remain under terror alert 3, it means that something is still up,” the interior minister, Jan Jambon, said. “Last night, we had a very successful action.”

The prosecutor’s statement said that “the results of the investigation necessitated an immediate intervention”, indicating a violent attack was likely planned in the near future.

The office did not link the raids to the attacks on 22 March, even though an eighth suspect was arrested as part of the investigation into those attacks late on Friday. A Belgian man, identified as Youssef EA, was charged with “participation in the activities of a terrorist group, terrorist murders and attempts to terrorist murders”.

Meanwhile, four senior ministers, including Michel, Jambon and the justice minister, Koen Geens, have received special protection following unspecified threats.

Geens said: “We learned about that late yesterday, that this close protection would happen. They say there are good reasons for that.”

Europe has been on high alert since the Euro 2016 football tournament began in neighbouring France earlier this month, and following the attacks by Islamist suicide bombers in Brussels in March and Paris in November.

The Flemish public broadcaster VTM said those arrested overnight were suspected of planning an attack in Brussels during one of Belgium’s Euro matches this weekend.

On Wednesday, Belgian police received an anti-terrorism alert warning that a group of Islamic State fighters had recently left Syria en route for Europe where they were allegedly planning attacks in Belgium and France, security officials said.

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