Spain has no "conclusive information" about the cause of a blackout that has disrupted the lives of millions across Spain, Portugal and France, Prime Minister Pedro Sanchez said on Monday.

"We still do not have conclusive information about the reasons for this outage... It's better not to speculate, we will know the causes, we do not rule out any hypothesis," Sanchez said in a televised address. 

Spain is aiming to restore power nationwide "soon" after a massive blackout affecting the Iberian Peninsula brought the country to a halt on Monday, Prime Minister Pedro Sanchez said.

Interconnections with neighbouring France and Morocco have already partially restored supply, while "combined cycles and hydroelectric plants throughout the country have also been reactivated, which should allow us to recover the supply across Spain soon", Sanchez said in a televised address.

Millions in Spain, Portugal and parts of southern France were affected by a rare power outage, bringing traffic, trains and regular activities to a halt, causing panic among people.

Initially, the cause of the outage was unknown, but the Portuguese power grid operator said a "rare atmospheric phenomenon" was behind the blackout, and it could take at least a week to fully restore electricity.

A nationwide power outage will prevent Spain from restoring inter-city train services on Monday, the transport minister said, as the crisis left thousands of passengers stranded.

"Today it is not expected that the circulation of medium and long-distance trains will be restored... We are working so that, once the electricity supply is restored, we can resume those services, which will no longer be possible today," Oscar Puente wrote on X. 

Play has been cancelled at the Madrid Open for the rest of the day following a massive power cut in Spain and neighbouring Portugal, tournament organisers announced Monday.

A power cut affecting "the whole of the Iberian peninsula and part of France", according to Portuguese electricity network operator REN, halted matches shortly after 10.30am GMT, before Madrid Open organisers called off the rest of the day's action four hours later. 

Spain's nuclear power plants automatically stopped on Monday when a 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.

That happened when the country suffered a national blackout at 1030 GMT, halting trains, fouling traffic and delaying flights.

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.

Electricity has returned to parts of northern, southern and western Spain, the grid operator said on Monday, as a blackout sparked chaos for millions across the Iberian peninsula.

"Tension has now been restored in substations in several areas of the north, south and west of the peninsula, beginning to give supply to consumers in these areas," Red Electrica said in a statement.

EU chief Antonio Costa said on Monday that "at this point, there are no indications of any cyberattack" after a massive blackout hit Spain and Portugal.

"Grid operators in both countries are working on finding the cause, and on restoring the electricity supply," Costa, who heads the European Council representing EU member states, wrote on X, after being in touch with leaders in Spain and Portugal.

A massive power cut in Spain and Portugal on Monday has disrupted air traffic, affecting flights to and from Madrid, Barcelona and Lisbon, European air traffic organisation Eurocontrol said.

The organisation said it was still too early to say how many flights would be affected.

It is still impossible to know how long Portugal will be without power after a massive blackout Monday that also affected Spain and parts of France, the country's electricity grid operator said.

"At this time it is still impossible to predict when the situation will be normalised," Portuguese operator REN said in a statement, adding it had "all resources deployed" to resolve the blackout.

Panicked customers scrambled to withdraw cash from banks and streets overflowed with crowds trying in vain to get a signal as a Spain-wide power outage plunged the country into chaos on Monday.

Carlos Condori, one of millions of people affected in Spain and Portugal, was travelling on the Madrid metro when the blackout brought his journey to a shuddering halt.

"The light went out and the carriage stopped," but the train managed to crawl into the platform, the 19-year-old construction sector worker told AFP outside a metro station in central Madrid.

"People were stunned, because this had never happened in Spain," he added. "There's no (phone) coverage, I can't call my family, my parents, nothing: I can't even go to work."

At Cibeles Square, one of the Spanish capital's busiest thoroughfares, the blackout of traffic lights unleashed a cacophony of sirens, whistles and car horns as police tried to control the pile-up of traffic.

Bewildered office workers congregated in streets with their computers made useless without internet,  alongside residents were thankful they had not been trapped in lifts.

Spain Power Outage Live Updates: People use their cell phones' flashlights in a dark sports shop after the Madrid Open matches get suspended due to a power outage

The Spanish grid operator has said at least six to eight would be needed to restore power after a massive outage that hit millions across Spain, Portugal and southwestern France.

The operator has urged citizens not to "speculate" on the cause of the blackout.

"We are looking at between six and 10 hours, if all goes well," Eduardo Prieto, head of Red Electrica, told Cadena Sur radio, adding that several parts of the network "are already repaired" and saying it was impossible "to speculate on the cause for the moment".

Power was restored in southwestern France after a blackout on Monday that also hit Spain and Portugal, French high-voltage grid operator RTE said, adding it remained unclear what caused the outage.

"An electrical incident is currently affecting Spain and Portugal, the cause of which remains to be determined," RTE said. "In France, homes were without power for several minutes in the Basque Country. All power has since been restored."

The European Commission said on Monday it was "in contact" with Spanish and Portuguese authorities "to understand the underlying cause" of a massive blackout that also hit part of southwest France. 

"The commission will keep monitoring the situation and make sure that there is smooth information exchange amongst all relevant parties," a spokesperson for the EU's executive arm said.

France mobilised to 'restore' power to Spanish network, the French operator said.

Spain Power Outage Live Updates: The Prime Minister's office said they are working to identify the origin of the blackout which has affected millions.

Many traffic lights had ceased to function, forcing vehicles to slow down to avoid collisions, while metros and trains were halted. Spain's national road authority, DGT, urged motorists to stop using the roads.

Spain Power Outage Live: The Madrid Open tennis tournament has been suspended due to power outage

The Spanish electricity grid said, "We are beginning to recover power in the north and south of the peninsula, which is key to gradually meeting the electricity supply."

"This process involves the gradual energization of the transmission grid as the generating units are connected. We continue working to restore supply," they added.

Spain Power Outage Live: The blackout was reportedly caused by the fire in the south-west of France which damaged a high-voltage power line but it is yet to be ascertained. 

Spain Power Outage Live: Rail operators said trains across Spain have stopped working because of the outage.