Police in Greece arrested four suspected terrorists Saturday, while in Belgium up to 300 troops were deployed to guard against attacks.
The arrests and troop deployments come after 13 people were detained, and two suspects killed, during counterterrorism raids in the cities of Verviers and Brussels on Thursday.
"Up to 300 troops will be progressively deployed," Prime Minister Charles Michel said in a statement. "The mobilized troops will be armed and their primary responsibility will be to survey certain sites."
The extra troops will be present in Brussels and Antwerp, and may be deployed to Verviers and other sites if needed.
The sensitive sites included the U.S. and Israeli embassies, as well as NATO and EU buildings, Reuters reported.
"It's very important to say that this wasn't a simple decision, but it was necessary, at a time when police are overly engaged, for the army to enter in a supporting role," Defense Minister Steven Vandeput told reporters.
Belgium's terror threat level has been raised to three, its second highest level, in the wake of the raids, which authorities said broke up plans for a "possible terrorist attack."
Five people were charged Friday with terrorist offenses.
"It was a possible terrorist attack, but we don't know if there were more, if this is really a network of terrorist cells in Belgium and if they're going to attack now, in the next days," Deputy Prime Minister Jan Jambon told the BBC World Service. "Now we have to use all the forces that we have to protect us."
"We've been investigating the data that we found in the houses that we searched, and we're investigating the suspects... and we're looking for other possible targets," Jambon said.
It comes after a series of deadly terrorist attacks in Paris earlier this month, which have put European security agencies on high alert.