"It is reasonable to assume... that further attacks are likely," Europol director Rob Wainwright told lawmakers in a hearing in the European Parliament in Brussels, Reuters wrote.
He compared Friday's events in Paris to those in Mumbai in 2008, when militants killed 166 people at different locations across the Indian city.
"The reality of what happened in Mumbai then has now arrived in Europe," Wainwright said.
"This is clearly therefore a more significant and threatening form of terrorism than the phenomenon of the lone actor," he added, referring to attacks over recent years by individuals or small groups inspired by Islamic State.
"It's also a clear statement of intent by ISIS (Islamic State) to export its brutal brand of terrorism to Europe to take it more onto the international stage."
Noting other attacks including last month's downing of a Russian airliner in Egypt, he added: "We are dealing with a very serious, well resourced, determined international terrorist organization that is now active on the streets of Europe. This represents the most serious terrorist threat faced in Europe for 10 years."
The hearing comes on the eve of an extraordinary meeting of EU interior and justice ministers to discuss measures after the attacks in Paris.
