"Today they hit Belgium, but they also hit the capital of the European Union," Renzi said in a speech, vowing that "it will take months, perhaps years, but we will beat them".
"This is not the time for jackals, but nor is it the time for doves. We need a European pact, a pact for freedom and security," he said, referring to those who used the explosions as an opportunity to attack the EU or simply urge peace.
"Europe must go all the way this time. We must invest in a common security and defence structure," he said, adding that the debate over defence integration has been raging since 1954, when a plan to form a European Defence Community (EDC) with a pan-Europe military fell through.
Further attempts have been thwarted by traditional "Nato-first" reflexes, a lack of political will, conservative defence industry policies and fragmentation of military cooperation, according to the European Political Strategy Centre.
"The security services must work together, and better together, with constant, timely and continuous cooperation," Renzi said, adding that "Italy unfortunately has experience to offer" -- a reference to Mafia and Red Brigades violence.
The battle against "a threat that is global but killers which are local" should be fought "with security forces but also primary school teachers... if we are to save the next generation," he said.
Belgium locked down its capital of Brussels, also home to the headquarters of the European Union and Nato, and imposed its highest level of security alert after the explosions.
The fact that extremists were able to hit high-profile targets in Brussels, Europe's symbolic capital, just months after Isis jihadists killed 130 people in Paris, raised fresh questions about the continent's ability to cope with the terror threat.