Belgian leaders on Friday sought sweeping new powers to monitor and punish their citizens for involvement with terrorism, joining France in an effort to rewrite laws just hours after dozens of arrests across Europe offered dramatic evidence of the threats security officials say are facing the continent.

The raids, carried out by France, Belgium and Germany on Friday to thwart unrelated terrorist plots, came after 17 people were killed in France last week in the worst terrorist attacks in that country in decades. The violence galvanized leaders and has swiftly led to grim discussions of crackdowns in nations that have long prided themselves on their willingness to live and let live.

Belgian leaders said Friday that they would seek to expand the list of offenses for which they could strip some people of their citizenship. France has fast-tracked the convictions of those accused of hate speech, handing down years-long prison sentences within hours of the initial offense. British Prime Minister David Cameron called for eagle-eyed surveillance of social networks.

Some of the counterterrorism proposals were already being discussed before two brothers claiming al-Qaeda ties stormed the offices of a Paris satirical newspaper Jan. 7, killing 12 people and initiating three days of deadly violence. But European leaders have since moved to expand those powers with a degree of political unanimity reminiscent of the ­bipartisan U.S. passage of the USA Patriot Act after the attacks of Sept. 11, 2001.

“At the present time, we do not have knowledge of a concrete and precise threat,” Belgian Prime Minister Charles Michel said Friday before unveiling 12 new measures intended to keep terrorism from Belgian soil. “We know that zero risk does not exist, neither in Belgium nor abroad,” he said.

European authorities widened crackdowns against suspected Islamist militant networks Friday. They arrested at least two dozen people, some of them on suspicion of aiding last week’s bloodshed in Paris and others for plotting separate attacks.

The sweeps started Thursday with a raid in eastern Belgium that left two suspected terrorist plotters dead and uncovered weapons and bomb material that officials said they believe were intended for a “major” attack that was foiled perhaps hours before it was to be launched.

In Brussels, officials announced that 13 people had been detained in Belgium and two in France over the course of the operation. They said they recovered four Kalashnikov rifles, a cache of small arms, explosives and several Belgian police uniforms.

Eric van der Sypt, a spokesman for the Belgian prosecutor’s office, said the uniforms were intended as disguises as part of a plot to kill police on the street or in their offices.

“We thought that the attack was imminent. It could have been hours or a day, certainly no more than a few days,” he said.

But, underscoring the elusive nature of an increasingly atomized and self-organized threat, he said he could not be certain whether the raids prevented or provoked further attacks.

“I have no idea if we diminished or put the level higher for a terrorist threat in Belgium,” van der Sypt said. He added that authorities believed there was no connection to the Paris attacks.

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. As many as 5,000 European citizens have gone to fight in Syria, European law enforcement officials say.

Apart from expanding powers to strip citizenship for dual nationals, Belgian leaders on Friday proposed devoting an additional $348 million to counterterrorism efforts. They also said they wanted the ability to take away identity documents to make it more difficult for people to travel to Syria and elsewhere.

The attacks have generated rare political accord over a need for broad new security steps in Belgium, a nation so fractious that 13 political parties sit in the national parliament.

“As a result of the events in Paris, combined with what happened yesterday in Belgium, the political unanimity is quite great,” said Rik Coolsaet, a terrorism expert at Ghent University. “It is a bit of 9/11 syndrome.”

France is also charging forward with attempts to expand government powers to monitor threats — and to punish those who praise or do not readily condemn terrorism. Leaders this week called for new legislation to significantly bolster domestic intelligence agencies.

Another law, a fast-track judicial process for accusations related to terrorism, was on the books as of November but had not been widely used before the Paris attacks. In recent days, however, prosecutors have filled the dockets with more than 100 cases that are speeding through courtrooms. People who have expressed support for the attacks have been sentenced to as much as 15 months in prison.

A top French opposition politician, Éric Ciotti, said this week that the government should withhold social benefits from the parents of children who failed to observe moments of silence in schools.

Also Friday, French police tried to sweep up the remaining support network of the three men who committed last week’s attacks in and around Paris.

In Paris, the prosecutor’s office said 12 people were detained in anti-terrorism raids overnight. Police questioned them about “possible logistical support” — mainly with arms and vehicles — to Amedy Coulibaly, the man suspected of killing a police officer before taking hostages at a kosher grocery store.

Berlin police, meanwhile, said 250 policemen raided 11 residences in and around the German capital overnight, taking two men into custody on suspicion of recruiting fighters and procuring equipment and funding for Islamist extremists in Syria.

Authorities said the raids were not connected to the French or Belgian networks. But they said they were working closely together.

“In matters of counterterrorism, coincidences don’t exist,” said Christophe Crepin, a spokesman for a French police union. “It’s pretty clear that police services across Europe are sharing information.”

Anthony Faiola and Stephanie Kirchner in Berlin, Daniela Deane in London, Virgile Demoustier in Paris and Hélène Bonaert in Brussels contributed to this report.