European computer guards battled Thursday against a simulated attempt by hackers to bring down critical Internet services in the first pan-continental test of cyber defences.

All 27 of the European Union's member nations as well as Iceland, Norway and Switzerland took part in the simulation as participants or observers, working together against the fictitious online assault, the European Commission said.

The exercise was based on a scenario in which one country after the other increasingly suffered problems accessing the Internet, making it difficult for citizens, businesses and public institutions to access essential services.

Security experts had to work together to prevent a simulated "total network crash," said Jonathan Todd, a commission spokesman for digital affairs.

"I would like to emphasise, so as to avoid any 'War of the Worlds' scenario here, this is purely an exercise and practice," Todd told a news briefing.

"There will be absolutely no effect on Internet connectivity in Europe."

The threat of assaults on computer systems has been identified as a key challenge in Europe, the United States, and NATO.

The threat came to life in a costly cyber strike against Estonia in 2007 and the Stuxnet computer worm attack in Iran this year.

The United States held its own major exercise against a large-scale cyber attack on critical infrastructure in late September with 12 international partners and 60 private companies.

The European exercise will be followed by more tests with more complex scenarios on the global level, the EU's executive arm said.

The EU hopes the exercise will help the bloc understand how such an incident can take place and ensure that authorities know who to contact in other members states in any cyber strikes, the commission said.

"This exercise to test Europe's preparedness against cyber threats is an important first step towards working together to combat potential online threats to essential infrastructure and ensuring citizens and businesses feel safe and secure online," said European technology commissioner Neelie Kroes.

The European Commission proposed on September 30 a new regulation that would impose heavier criminal sanctions against hackers and producers of malicious software.

Cyber security will be one of the top issues that NATO leaders will tackle at a summit of the 28-nation military alliance in Lisbon no November 29-20.

During a visit to Brussels in mid-September, US Deputy Defense Secretary William Lynn urged NATO allies to build a "cyber shield" against the treat of computer attacks.

Warning that military assets were not the only targets of cyber strikes, he underscored the need to protect the computer systems of vital economic infrastructure such as power grids and financial markets.