Belgian police raided an Islamist cell planning attacks against police on Friday as dozens of people were arrested in sweeps across Europe, keeping the continent on alert one week after the Paris attacks.
Two suspected jihadists were shot dead in a police raid in the eastern Belgian town of Verviers on Thursday night and prosecutors said 13 suspects had been detained across Belgium, with two more held in France.
French police separately detained 12 people in the suburbs of Paris in connection with last week's attacks on the Charlie Hebdo satirical magazine, a Jewish supermarket and a policewoman, in which 17 people were killed.
Hundreds of German police meanwhile raided alleged Islamist sites in Berlin, arresting two men suspected of being part of a group planning to carry out an attack in Syria.
The raids highlighted fears about young Europeans travelling to fight with the Islamic State and Al-Qaeda-linked groups in the Middle East before returning to carry out attacks on western targets.
"The group was on the verge of carrying out terrorist attacks to kill police officers in public roads and in police stations," Belgian federal prosecutors' spokesman Eric Van der Sijpt told a news conference about the raids overnight.
Police found Kalashnikov assault rifles, explosives, ammunition and communications equipment, along with police uniforms that could have been used for the terror plot, he said.
Members of the group had recently returned from Syria, prosecutors said, but they said there still appeared to be no direct link to the Paris attacks.
Prime Minister Charles Michel raised Belgium's terror alert to its second highest level after the raids. The European Commission stepped up security at its headquarters in Brussels as a "precaution", a spokeswoman said.
Jewish schools in Brussels and the port city of Antwerp closed Friday.
The unrest across Europe has fuelled fears of tensions between communities, but in Verviers, a faded industrial town near the German border, residents said they would stay united.
"The Muslim community had the strongest reaction, one that says that none of this should exacerbate the delicate balance that makes this town stand on its feet," mayor Marc Elsen said.
Belgium has one of the largest number of extremists who have returned from Syria relative to its population, with a large Muslim community that suffers from high unemployment and disenfranchisement.
With France still reeling from the attacks which targeted its cherished traditions of free speech, US Secretary of State John Kerry laid wreaths on Friday at both Charlie Hebdo offices and the Jewish supermarket during a visit to Paris.
It follows criticism of the US for not sending a top representative to a march in Paris on Sunday, which drew 1.5 million people and dozens of world leaders in the wake of the attacks.
The magazine inflamed Muslims in many countries by printing cartoons of the Prophet Mohammed.
The funeral of Stephane Charbonnier, alias Charb, the editor-in-chief of Charlie Hebdo, was also due to take place on Friday.
The nine men and three women arrested in France overnight were to be questioned about "possible logistic support" they may have given to the gunmen -- Islamist brothers Said and Cherif Kouachi and Amedy Coulibaly -- in particular weapons and vehicles, the source said.
In Germany an alleged leader of a group planning to carry out an attack in Syria and the man in charge of financing were arrested in raids on suspected Islamist sites in and around Berlin by more than 200 police officers, officials said.
They were suspected of leading an Islamist group of "Turkish and Russian nationals from (the Caucasus regions' of) Chechnya and Dagestan."
International anger over Charlie Hebdo's printing of a new image of Mohammed in its sold-out comeback issue this week continued to rage, with protesters clashing with police outside the French consulate the Pakistani port city of Karachi.
AFP photographer Asif Hassan was shot while covering the demo but appeared to be out of danger following surgery.