BARCELONA, Spain >> An unprecedented blackout brought much of Spain and Portugal to a standstill Monday, stranding thousands of train passengers and leaving millions without phone and internet coverage and access to cash from ATMs across the Iberian Peninsula.

The sudden crash of the power grid also left authorities searching for its cause. Spain’s Prime Minister Pedro Sánchez addressed the nation and said that almost 11 hours after the nation ground to a halt, government experts were still trying to determine what happened.

“We have never had a complete collapse of the system,” Sánchez said, before detailing that at 12:33 p.m. Monday Spain’s power grid lost 15 gigawatts, the equivalent of 60% of its national demand, in a matter of five seconds.

Spanish power distributor Red Eléctrica’s head of operations Eduardo Prieto said the event was “exceptional and extraordinary.”

Spain had recovered nearly 50% of its power by 11 p.m., and the prime minister pledged that the entire country of 48 million would have lights back on by the end of Tuesday.

It was the second serious European power outage in less than six weeks after a March 20 fire shut down Heathrow Airport in the U.K., and it came as authorities across Europe gird against sabotage backed by Russia.

The Portuguese National Cybersecurity Center in a statement said there was no sign the outage was due to a cyber attack. Teresa Ribera, European Commission executive vice president in charge of promoting clean energy, indicated the same to journalists in Brussels and called the power outage “one of the most serious episodes recorded in Europe in recent times.”

“We are analyzing all the potential causes without discarding any hypothesis,” Sánchez said.

The outage began at midday. Offices closed and traffic was snarled in Madrid and Lisbon, while some civilians in Barcelona directed traffic. Train services in both countries stopped.

Emergency services and rail workers in Spain had to help evacuate some 35,000 people from over 100 trains that stopped on the tracks when the electricity was cut. By 11 p.m. passengers from 11 trains still needed evacuating, Sánchez said.

The subway systems shut down.

“I don’t know how I am going to get home,” Barcelona resident Ivette Corona told The Associated Press she watched a large group of people fail to get on a bus that briefly stopped to squeeze in a couple of passengers.

Hospitals and other emergency services switched to generators and gas stations stopped working. It wasn’t possible to make calls on most mobile phone networks, although some apps were sporadically working. People searched for battery-powered radios.

It is rare to have such a widespread outage across the Iberian Peninsula, with a combined population of about 60 million. Spain’s Canary Islands, Balearic Islands and the territories of Ceuta and Melilla, across the Mediterranean in Africa, were not affected.

After an extraordinary meeting of the National Security Council, Sánchez said the army would distribute generators and other material to the hardest-hit areas Tuesday.

The Portuguese Cabinet convened an emergency meeting at the prime minister’s residence. Portuguese Prime Minister Luis Montenegro said he had spoken several times to Sánchez and expected power to be restored by the end of the day.

Portugal’s government said the outage appeared to stem from problems outside the country, an official told national news agency Lusa.

Electricity was being pulled from Morocco and France to restore power to southern and northern Spain, Spain’s prime minister said, thanking their governments. Spain was also increasing the production from hydroelectric and combined cycle thermal power plants.

A graph on Spain’s electricity network website showing demand across the country indicated a steep drop around 12:15 p.m. from 27,500 megawatts to near 15,000 megawatts.

Spanish airports were operating on backup electrical systems and some flights were delayed, according to Aena, which runs 56 airports in Spain including Madrid and Barcelona.

In Lisbon, terminals closed and tourists sat outside waiting for news about flights. “We haven’t seen any plane arriving or departing in the 50 minutes we’ve been waiting here,” Dutch tourist Marc Brandsma told the AP.

The Spanish Parliament in Madrid closed. Play at the Madrid Open tennis tournament was suspended.