Troops fanned across Belgium on Saturday and Sunday to protect high-profile targets. (Reuters)

Belgian authorities are still pursuing the mastermind of a foiled plot to kill police officers, Belgian Justice Minister Koen Geens said Sunday, as hundreds of Belgian soldiers fanned out across the country to protect high-profile targets.

Belgian police released several of the 13 people who were detained last week as part of an effort to stop an attack that security officials said was hours or days away from happening. But the ringleader was still on the run, Geens said, underlining the challenge European nations face in policing their large, borderless territory.

European Union foreign ministers plan to meet Monday to discuss the rising threat of homegrown Islamist militancy, which was thrown into relief after three Frenchmen claiming ties to al-Qaeda and the Islamic State killed 17 people in and around Paris over three days of violence early this month. Belgian authorities said they halted a separate plot last week connected to returnees from the fighting in Syria, which has turned into a worldwide magnet for militants seeking to establish an Islamic caliphate.

In Brussels, further raids took place Sunday connected to the Belgian attack attempt, although no arrests were made, a spokesman for the Belgian federal prosecutor said. The raids came as 300 soldiers were posted around the country to protect potential targets of attack, according to a statement from the office of Prime Minister Charles Michel.

Greek police said a day earlier that they had detained several people in Athens on suspicion that they were connected to the Belgian plot. Sunday evening, Belgian prosecutors asked for one person to be extradited, Reuters reported.

“The arrests that took place yesterday evening were unable to apprehend the right person. We continue to search actively, and I expect it will succeed,” Geens told the broadcaster VRT.

Belgium’s Derniere Heure newspaper reported that police are seeking a Belgian man of Moroccan background named Abdelhamid Abaaoud, 27, also known as Abou Omar Soussi, who traveled to Syria to join the Islamic State. He has been known to authorities for some time and was featured in March in a video that showed him driving a car in Syria with a dead body dragging behind it.

Belgian authorities have refused to confirm that they are seeking Abaaoud.

Separately on Sunday, Italian Interior Minister Angelino Alfano said that his country had expelled nine alleged Islamist militants since late December and that it expected to kick out more.

“Those expelled . . . were here for years, and two of these involved their respective families, sending them into Syria to fight,” Alfano said, according to the Associated Press. He said 59 people had traveled from Italy to Syria to fight.

The attacks this month have quickly polarized Europe, as politicians vow a crackdown and Islamic communities say they are being unfairly targeted for the violence of an extremist few.

French authorities banned an anti-Islamist rally intended for Sunday, fearing that it could stir violence. In Germany, a weekly anti-Islam protest that has been drawing tens of thousands of supporters was called off for Monday after an organizer received death threats. The group, known as Pegida, or Patriotic Europeans Against the Islamization of the West, has held weekly protests this month in the eastern city of Dresden that have grown steadily larger.