European computer guards joined forces Thursday to counter a simulated attack by hackers on critical Internet services, in the first pan-continental test of cyber defences.

The day-long exercise was to simulate a gradual deterioration of Internet connections in one country after another and, in a worst-case scenario, if all major cross-country connections were out of use.

European citizens, businesses and public institutions would have trouble accessing essential online services unless experts were able to reroute the traffic from affected inter-connections, the European Commission said.

The countries participating in the simulation have to work together to prevent a simulated "total network crash," the European Union executive said.

All members of the 27-nation European Union as well as Iceland, Norway and Switzerland were taking part in the simulation as active participants or observers.

"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 simulation will be followed by more tests with more complex scenarios on the global level, the European Commission said.