Belgian counterterrorism police conducted raids across their nation Thursday, killing two suspected Islamist militants and disrupting an alleged plot to launch an attack that would have been the second instance of homegrown Islamist violence in Europe in just eight days, officials said.

The dramatic police strikes coincided with heightened alerts across Europe in the wake of the deadly attacks in Paris last week by men claiming ties to al-Qaeda and the Islamic State. European counterterrorism officials have been warning that their top security threat is the risk of attacks by their own citizens, radicalized by the conflict in Syria. The Belgian and French events, which did not appear to be directly tied to each other, underscored the dangers facing the continent.

A spokesman for the Belgian national prosecutor’s office, Eric Van der Sypt, told reporters that counterterrorist police raided a cell of returnees from Syria who were planning “a major imminent attack” in Belgium. Two of the suspects died in a shootout, and a third was severely wounded, he said.

Van der Sypt said the suspects, armed with what he described as weapons of war, immediately opened fire on security forces who carried out the operation in the town of Verviers, about 75 miles east of Brussels. Other raids were carried out in the Brussels region, he said, and Belgium raised its terror alert to its second-highest level as the operations continued. Police were issued 10 search warrants in all, he said.

The target of the alleged terrorist plot was not immediately clear, although Brussels’s Le Soir newspaper reported that it may have been the police. Videos of the confrontation — which started at 6 p.m., near the height of rush hour — show explosions, gunfire and flames at a building near the Verviers train station.

An investigation found “several people who we think are an operational cell, certain people who came back from Syria,” Van der Sypt said. “During the investigation, we found that this group was about to commit terrorist attacks in Belgium.”

There were no casualties among the security forces in the raid, he said.

Additional raids were carried out in the capital, Brussels, where sirens could be heard wailing across the city, and near the suburb of Vilvoorde, which has become a haven for recruiters seeking to send fighters to Syria.

Belgium’s VRT radio and television network reported Thursday that Belgian security authorities had been monitoring the Verviers suspects for at least two weeks, before the attacks in Paris set off a wave of fears over homegrown Islamist militancy within Europe. Police recovered four Kalashnikov rifles, bombmaking materials and police uniforms from the Verviers raids, Le Soir reported.

The newspaper also reported that a man was seen earlier Thursday brandishing a weapon and chanting religious phrases in Arabic and French in a Brussels subway station and was detained.

The affiliation of the suspected terrorist cell in Verviers was not disclosed, but the radical Islamic State has captured large swaths of territory in Syria and neighboring Iraq and has sought to recruit Westerners to fight for the creation of an Islamic caliphate.

According to security officials in Belgium, more than 350 Belgians have gone to Syria to join the fighting there, the highest number per capita among European countries and a shock for the nation of 11 million people.

Video captured by a witness shows a Belgian counterterrorist police raid on an apartment used by suspected Islamist militants. (Reuters)

In May, a French citizen who had returned from Syria attacked the Jewish museum in Brussels, killing four people in what was said to be the first attack by a Syrian returnee on European soil. Many Belgian Jewish schools announced they would stay closed Friday, a measure of the fears after the attacks in France, one of which was against a kosher grocery store.

Earlier Thursday, prosecutors said they had detained a man in Belgium whom they suspected of supplying weapons to Amedy Coulibaly, a French citizen who killed four people and took hostages at that store in eastern Paris last week. Coulibaly was killed later that day in a police raid on the store that coincided with an assault on the hideout of two brothers who massacred 12 people in the offices of the satirical newspaper Charlie Hebdo in Paris. The two brothers, Said and Chérif Kouachi, died in the police assault about 25 miles northeast of Paris.

The three gunmen killed a total of 17 people in last week’s violence in France, which began with the storming of the newspaper on Jan. 7. At least one of the two brothers in the Charlie Hebdo attack went to Yemen in 2011 to link up with al-Qaeda’s affiliate there.

The affiliate in Yemen claimed it helped plan and fund the attacks.

Before attacking the kosher grocery store, Coulibaly had made a video in which he said he was acting on behalf of the Islamic State. Hayat Boumeddiene, his companion and suspected accomplice, fled Europe last week for Syria, Turkish officials have said.

Thursday’s raids came hours after French President François Hollande and German Chancellor Angela Merkel gave speeches in defense of Islam and religious freedom in their countries. Muslim communities across Europe have come under intense pressure from far-right nationalist parties after the Paris attacks.

French security officials, mean­while, said they were facing an unprecedented onslaught of hacking attacks on the Internet — more than 19,000 incidents in the past eight days.

Charlie Hebdo’s first post-attack edition, released Wednesday, featured a cartoon of a tearful Muhammad on its cover. Many Muslims find the depictions of the prophet insulting.

Pope Francis on Thursday said he sympathized with those who were offended.

“You cannot insult the faith of others,” he told reporters on his plane. “It’s true that you can’t react with violence.” But if a friend “insults my mother, he’ll get a punch. It’s normal,” he said.

Brian Murphy and William Branigin in Washington contributed to this report.