Millions of people have been impacted by nationwide power outages across Spain and Portugal that have paralysed public transport, caused large traffic jams and delayed flights.

Both countries were plunged into mayhem when an unknown cause shut down the electricity networks on the Iberian Peninsula.

Traffic lights stopped working, transport networks were halted, hospitals were left without power and people were trapped in the metro and in elevators.

In Madrid, hundreds of people stood in the streets outside office buildings.

There was a heavy police presence around some important buildings, with officers directing traffic as well as driving along central atriums with lights.

Spain on Tuesday morning (AEST) declared a state of emergency and pleaded with the public to stay off the roads.

Conspiracy theories have been circulating as utility operators rushed to work out the problem and restore the grid.

Officials said the possibility that it was caused by a cyber attack had not been ruled out.

However, Portuguese grid operator REN said a rare atmospheric phenomenon in Spain, due to extreme temperature variations in the country’s interior, caused the power outages.

REN said that fully restoring the country’s power grid could take up to a week.

Spain’s Red Eléctrica said the outage could last from six to 10 hours.

The Spanish and Portuguese governments met to discuss the outage, which also briefly affected parts of France, and a crisis committee was set up in Spain, sources familiar with the situation said.

Spanish Prime Minister Pedro Sánchez visited electricity transmission operator Red Eléctrica’s control centre.

“The government is working to determine the origin and impact of this incident and is dedicating all resources to resolve it as soon as possible,” the Spanish government said.

Red Electrica said it was working with regional energy companies to restore power. REN also said it had activated plans for the phased restoration of the electricity supply across Portugal.

Play at the Madrid Open tennis tournament was suspended, forcing 15th seed Grigor Dimitrov and British opponent Jacob Fearnley off the court as scoreboards went dark and overhead cameras lost power.

The European Commission said it was in contact with the authorities in Spain and Portugal and the European network of transmission system operators ENTSO-E to try to establish the cause of the outage.

Power outages on this scale are rare in Europe. In 2003, a problem with a hydroelectric power line between Italy and Switzerland caused a major outage across the whole Italian peninsula for around 12 hours.

In Madrid, the air was filled with the sound of police sirens and helicopters clattered overhead.

The towering Torre Emperador skyscraper in the Spanish capital was evacuated via stairs. Worried people tried desperately to reach their children’s schools as the cell signal came and went.

In a video posted on X, Madrid Mayor Jose Luis Martinez-Almeida urged the capital’s residents to minimise all travel and stay at their current locations if possible.

Airports reported delays. AENA, which manages 46 airports in Spain, reported flight delays around the country.

Portugal’s airport operator ANA said airports activated emergency generators which for now allows essential airport operations to be maintained at Porto and Faro airports.

“In Lisbon, operations are ongoing but with limitations. So far, there have been no impacts on Madeira and Azores airports,” it said.

-with AAP/AP and DPA