Only the island of Sardinia was unaffected by the nationwide outage, which left about 50 million of Italy's 57 million inhabitants without power from about 3:30 am (0130 GMT) and up to 30,000 passengers blocked in trains across the country.

Hospitals nationwide switched to emergency generators following the cut, which came just six weeks after the worst blackout in American history and also followed outages in London as well as Sweden and Denmark.

In Milan and Rome, passengers were trapped in subway tunnels as escalators and lifts ground to a halt.

Television footage showed people blundering around in the dark, notably in the capital where ironically, a so-called "White Night" of round-the-clock cultural festivities -- an extravaganza of art events -- turned into a black night for millions.

RAI Uno television gave live coverage of the outage, showing images of Saint Peter's Basilica in the Vatican steeped in darkness.

A spokesman for the Italian national grid said it had lost control of the network in just four seconds, leading to a disastrous "domino effect" that cut off electricity nationwide.

It was still too early to give an exact explanation for the outage, he said, although early reports said the Italian grid suddenly lost 6,000 megawatts of power coming from France. A French official strongly denied responsibility for the blackout.

Power had been restored to northern Italy by around 8:00 am, including in the cities of Turin and Milan, two of the country's economic powerhouses, while lights flickered back on and elevators whirred back to life in Rome around midday.

In the capital citizens were advised to use power with moderation, while electricity to much of the south was still largely out. In 1994 18 million people in the south were left without power in the previously worst blackout in Italian history.

The head of the civil protection services, Guido Bertolasi, told RAI Uno following an emergency meeting that the situation was "under control", and that it was hoped the power supply could be fully restored by early afternoon.

"We immediately guaranteed power supply to essential infrastructures: hospitals, fire services and police. This is a serious problem -- fortunately it happened on a Sunday," he said.

Bertolasi also ruled out sabotage as the cause of the blackout.

People were also advised not to use their cars until traffic lights had been restored, nor to make unnecessary phone calls for fear of overloading the system.

Italian Industry Minister Antonio Marzano said the "exceptional incident" had been caused by an outage of two power lines carrying electricity from France to Italy.

"The truth is we are structurally dependent on electricity imports day and night," explained Carlo Bolino, the head of the Italian electricity network.

France's national grid confirmed early Sunday that two high tension powerlines supplying Italy were cut briefly early Sunday but denied responsiblity for the subsequent blackout.