Main and essential web services in European countries have come under simulated attack as they test their cyber defence wall.
This is the first-ever cross-European simulation. It was planned to see how can overcome such situations as the attacks slow connections.
The exercise drill 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 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.
A report on this exercise is due to be published on 10 November.
- BBC Inputs