European police arrested over two dozen suspects in anti-terrorism raids Friday when French, German and Belgian police rushed to stop terrorist attacks by people linked to Islamic extremists in the Middle East.

According to Rob Wainwright, the head of the police agency Europol, thwarting terror attacks has become "extremely difficult" because Europe's 2,500-5,000 radicalized Muslim extremists have little command structures and are increasingly sophisticated, Associated Press reported. 

Belgian investigators said a plot to murder police officers across the country has been foiled within days of being launched, according to Reuters. Two gunmen opened fire on police during the raid in Belgium and were fatally shot. The two Islamist extremist suspects are believed to have fought in Syria before returning to the European nation. Fifteen other suspects were arrested Friday.

"This group was on the point of carrying out terrorist attacks aiming to kill police officers in the streets and in police stations," state prosecutor Eric Van Der Sypt told reporters at a news conference.

Van Der Sypt say the suspects were set to act "in the next hours or days."

Prime Minister Charles Michel in Belgium urged people not to panic, insisting that authorities have thwarted any imminent attack.

There is no link between the Belgians killed Friday and the attacks in Paris that killed 17 people. Meanwhile, two other people were arrested in France.

After Secretary of State John Kerry arrived in Paris to offer U.S. solidarity in combating militants, a man took hostages with a military weapon at a post office near the city. He eventually surrendered. Paris also evacuated its busy Gare d L'Est train station on Friday but no bomb was found, according to the AP. In the raid, authorities also found several police uniforms which could have been used for terrorists to pass off as officers.

Authorities continue to search for more suspects as they cannot confirm that they have arrested everyone in the terror group.