Spain aims to restore power nationwide "soon" after a blackout disrupted daily life for millions but had no "conclusive information" about its causes, Prime Minister Pedro Sanchez said.

The Socialist leader called on people to show "responsibility and civility" to help authorities manage the crisis that also affected Portugal and France.

Interconnections with France and Morocco have already partially restored electricity to parts of northern and southern Spain, Mr Sanchez said in a televised address after an emergency government meeting.

"Combined cycles and hydroelectric plants throughout the country have also been reactivated, which should allow us to recover the supply across Spain soon," Mr Sanchez said.

"We still must focus on the most important thing: restoring electricity to our homes."

Transport ground to a halt after the outage struck at around midday. The transport minister announced that inter-city trains would not run again before tomorrow at the earliest.

Mr Sanchez said port and air traffic was unaffected, although flights were reduced by 20% "to ensure safety and smooth operation".

Hospitals continued to function thanks to generators that have "a long autonomy", he added.

Urging the population to minimise travel and telephone calls and follow information from official sources, Mr Sanchez appealed "to citizens to cooperate with all authorities, to act with responsibility and civility".

"Let's not spread or pay attention to information from dubious sources, as has happened in other crises," he said, warning against exacerbating the "anxiety" millions of people were going through.

Meanwhile, Portuguese grid operator REN has said that fully restoring the country's power grid could take up to a week.

The estimate came after it said a rare atmospheric phenomenon in Spain due to extreme temperature variations in the country's interior caused power outages across the Iberian Peninsula.

REN said extreme temperature variations in the interior of Spain caused anomalous oscillations in the very high voltage lines. It said this phenomenon is known as "induced atmospheric vibration".

The oscillations caused synchronisation failures between the electrical systems, REN said, leading to successive disturbances across the European network.

Watch: Traffic chaos in Madrid following massive power outage

Spain's nuclear power plants automatically stopped when the massive blackout hit, but diesel generators were keeping them in "safe condition", officials said.

The shutdown of the country's nuclear plants was "in line with their design" when confronted with an unexpected power outage, the Spanish Nuclear Safety Council (CSN) said in a statement.

Four reactors at three nuclear power plants were in operation when the outage struck and immediately went offline.

Reactors at two other nuclear plants were already stopped before the blackout and generators were also maintaining basic functions safely, the CSN said.

Small number of flights cancelled at Dublin Airport

A small number of flights have been cancelled at Dublin Airport as a result of the power outages in Spain and Portugal and a number of other flights from the countries are delayed.

Five flights to and from the Spain and Portugal have been cancelled while a further six are experiencing delays of between 30 minutes and three hours

The operator, daa said passengers should check with their airlines before travelling.

Utility operators sought to restore the grid but Spanish electricity transmission operator Red Eléctrica said the outage could last from six to 10 hours.

The outage caused chaos in parts of Portugal and Spain as traffic lights stopped working, causing gridlock.

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

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.

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.

Gridlock

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 mobile phone 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.