Spain has declared a state of emergency as large areas face a night without power after a mystery outage hit Spain and Portugal on Monday, paralysing transport networks and disrupting mobile communications.

On Monday, Spanish Prime Minister Pedro Sánchez said experts were still looking for the cause of the countrywide outage, which immobilised the rail system, delayed flights and made traffic lights go dark.

Speaking later, at 11pm local time, Sánchez said power had been restored to only half the country more than 11 hours after the electricity network collapsed.

Spain is one of the countries at the forefront of efforts to rely more on renewable electricity as part of the shift away from fossil fuels, but Monday’s crisis is likely to stoke concerns about power networks’ ability to meet the added demand and increased volatility of supply from renewables.

Elaborating on the possible cause of the blackout, Sánchez said: “At 12:33 this morning, 15GW of generation were suddenly lost from the system and they were lost in just five seconds. This is something that has never happened before . . . 15GW is equivalent to approximately 60% of the country’s demand at that time.

“The 60 per cent that caused this sudden disappearance of supply is something the specialists have not yet been able to determine, but they will and the relevant state institutions and all the private operators are working in a co-ordinated way to understand what has happened.

“All potential causes are being analysed . . . without ruling out any hypothesis, any possibility.

“The situation on the streets continues to be one of a return to normality.”

Weather data showed that the temperatures over southern Spain had risen sharply between midday and 1pm local time. Higher temperatures can limit how much electricity cables can carry.

However, Stephen Jarvis, an assistant professor in environmental economics at LSE, said the weather had not been abnormal and a system failure was the likelier culprit, as supply and demand became mismatched.

Eduardo Prieto, director of services at Spain’s operator, Red Eléctrica, said a “very strong oscillation” in the network during that period had cut off Spain’s grid from the rest of continental Europe, which had led to the system collapsing.

Red said that, at 10.30pm local time, it was meeting almost half of demand, partly thanks to supplies from France. Two-thirds of substations were working and it would “progressively increase” supply to normal levels.

In response to the declaration of the state of emergency, the national government said it would provide support to regions that requested it, and by late evening this included the capital Madrid, Murcia, Andalucia and Extremadura.

“We are going to go through some critical hours before we totally recover electricity,” Sánchez said after a crisis meeting in the late afternoon with top government officials.

In an address to the nation, he called on people to minimise travel, pay attention to official statements rather than social media and restrict their use of mobile phones. He also noted the outage’s “tremendous impact” on everyday life as well as economic loss and “anxiety in millions of homes”.

Sánchez said he was in contact with Spain’s King Felipe, political leaders and the EU and Nato.

Spain’s chief traffic authority called on people not to drive, because traffic lights were out of operation. The government added that medium or long-distance rail transport would not resume during the day.

Sánchez said about 35,000 passengers had been rescued from more than 100 trains that had stalled but that people were still stranded on 11 trains after more than 11 hours.

Earlier in Madrid, people spilled out on to the streets, as metro stations were evacuated and shops, restaurants and offices closed. Mobile phone coverage was also initially hit. Local media said some — but not all — hospitals were functioning as normal with the aid of backup generators.

Isabel Díaz Ayuso, conservative leader of the Madrid regional government, had called on Spain’s Socialist-led national administration to activate emergency plans “to allow the army to keep order, if necessary”.

Alberto Núñez Feijóo, head of the opposition People’s party, criticised Sánchez’s government for being slow to provide updates. “We need timely information,” he said.

At Madrid and Lisbon airports hundreds of flights were delayed and cancelled.

Aena, which runs Madrid airport and 45 others across Spain, said it was relying on backup power supplies to operate, adding that the extent of the delays would depend on whether crews and passengers could reach the airports.

Spain sources 43 per cent of its electricity from wind and solar power, but grid and storage capacity has not kept pace with rapid development of renewable energy.

Phil Hewitt, director at energy data specialist Montel, said the combination of Spain’s high use of wind and solar and its limited connections to neighbours could have made it hard to deal with the power fluctuations Spanish authorities cited in Monday’s outage.

The country has long lamented being an “energy island” because of its poor connections with France.

French grid operator RTE said parts of France had been briefly affected by the outage but that power had been quickly restored.

Additional reporting by Philip Georgiadis and Kenza Bryan