Essential web services have come under simulated attack as European nations test their cyber defences.
The first-ever cross-European simulation of an all out cyber attack was planned to test how well nations cope as the attacks slow connections.
The simulation steadily reduced access to critical services to gauge how nations react.
The exercise also tested how nations work together to avoid a complete shut-down of international links.
Neelie Kroes, European commissioner for the digital agenda, said the exercise was designed to test preparedness and was an "important first step towards working together to combat potential online threats to essential infrastructure".
The exercise is intended to help expose short-comings in existing procedures for combating attacks. As the attacks escalated, cyber security centres had to find ever more ways to route traffic through to key services and sites.
The exercise also tested if communication channels, set up to help spread the word about attacks, were robust in the face of a developing threat and if the information shared over them was relevant.
Overseeing the exercise was the European Network Security Agency which has been given new powers to help member states handle cyber security incidents. In all, 22 member states plus Iceland, Norway and Switzerland took part.
A report into how the simulation went and how different nations coped is due to be published on 10 November.