The European Union is hosting what it describes as its biggest cybersecurity exercise, BBC News reported.
Governments, businesses and ISPs (internet service providers) are being faced with 1,200 separate incidents during a simulated DDoS (distributed denial of service) attack.
A similar event was staged in 2010, but this is the first time that the bloc's banks have been involved. The results will be used to find ways to improve co-operation.
Enisa (European Network and Information Security Agency), which is co-ordinating the event, said 25 nations were actively participating in the practice run, and a further four countries were observing. But it would not specify the names of the states or organisations involved.
The event centres on a DDoS attack in which a mass of third-party computers attempt to overwhelm their targets' servers, forcing their websites offline and potentially disrupting their operations.
This type of attack has been used over recent months by members of the Anonymous hacktivist collective and others as a form of protest against companies and authorities whose actions they dislike.
Attackers used thousands of hijacked PCs to create several botnets using different types of DDoS attacks to consume all of the bandwidth available for the companies' customer-facing websites, making them inaccessible.
According to Enisa, reported web-based attacks increased by 36% over 2011. A World Economic Forum report estimated there was a one-in-10 risk that a "critical information infrastructure incident" could cause 200bn euros ($260bn; £160bn) of economic damage within the next decade.
But one security adviser to the British government questioned how much use the event would be against future cyber-assaults.
It's a very worthy exercise but it doesn't guarantee security," said Alan Woodward, a visiting professor at the University of Surrey.
"Penetration-testing assumes you will be attacked using existing techniques, but it doesn't necessarily reveal vulnerabilities you would otherwise not have known about. It's rather like checking your front door's locks are working, which doesn't reveal if your windows are open."
