A sweeping power outage left millions of people in Spain, Portugal and parts of France without electricity on Monday, leading to chaotic scenes and widespread disruption.

A problem with the power connection between France and Spain was a contributing factor to the outage, the head of a trade body that represents Europe’s power industry said.

Kristian Ruby, of Eurelectric, told the BBC Radio 4’s The World Tonight a “specific incident with an interconnector between France and Spain” occurred on Monday.

Mr Ruby said this “meant the Spanish grid was disconnected from the broader European grid”.

He added that the interconnector incident was unlikely to have caused the outage on its own and that there were “likely to have been other elements in this equation”.

A day of widespread disruption saw trains cancelled and evacuated in some regions, and traffic lights stop working as delays mounted at airports.

As of Monday evening, 11 trains remained stranded, Spanish Transport Minister Oscar Puente said.

With shops, homes and restaurants plunged into darkness – and some people stuck in lifts – Spain’s electricity network said by mid-afternoon that restoring power could take several hours.

By Monday night, 50% of power had been restored across Spain, according to Spanish Prime Minister Pedro Sánchez. While Portuguese energy provider REN said electricity had been restored to 750,000 customers.

But a state of emergency remained in place, with regions able to request the special status.

Sánchez said the cause of the outage was still being investigated and warned against speculation, while Portuguese Prime Minister Luís Montenegro said there was “no indication” of a cyber attack.

The first reports of an outage began around midday (10:00 GMT) on Monday and its impact quickly escalated.

As the scale of the disruption became clear, residents of Madrid were warned to stay put, keep off the roads and not to call emergency services unless “truly urgent”.

A Spanish operator said power restoration would take up to 10 hours, while Portugal’s power firm REN said getting back to full power could take up to a week.

Queues formed at cash machines as card payments were affected by the outage, and there were reports that some petrol stations are closed.

When the blackout hit the Madrid metro station network, commuters were left confused and panicking. One resident, Sarah Jovovich, described the sense of confusion underground when the lights went off in a metro station.

People were “hysterical” and “panicking”, she told the BBC. “It was quite chaotic really.”

Mobile phones had stopped working and nobody had any information, she said.

When she left the metro station and emerged into the streets, she found the roads gridlocked with heavy traffic. “No-one understood anything. Businesses were closed and buses were full,” she said.

The outage also left a number of people stuck in lifts.

Emergency workers were called to 286 buildings to free people trapped inside elevators in the Madrid region, the head of the regional government, Isabel Díaz Ayuso, told the television station Antena 3.

Spanish media reported that some hospitals had implemented emergency plans, including halting routine work, news agencies reported.

By early evening and after several hours of blackouts, residents in some parts of the country reported the lights were back on.

Power was being restored “in several areas of the north, south and west of the [Iberian] peninsula”, the Spanish grid operator said.

The authorities in Spain and Portugal were on Monday night still trying to work out what caused the power cuts.

“There are no indications of any cyberattack” at this point, the President of the European Council, Antonio Costa, said.

As panic spread earlier in the day, residents of Madrid were warned to stay off the roads.

In a video on social media, the mayor of the Spanish capital, José Luis Martínez-Almeida, told residents to “keep their movements to an absolute minimum and, if at all possible, to remain where they are. We want to keep all roads clear.”

People should only call emergency services if it is “truly urgent”, he said.

“If emergency calls go unanswered, go to the police and the fire stations in person, where they will try to deal with all the emergencies.”

The country’s nuclear power plants automatically stopped when the blackout hit, and the Spanish oil company Moeve said it halted operations at its oil refineries.

People were left stranded on trains in difficult to access areas, with Transport Minister Puente saying there were issues with coupling locomotives.

Speaking to the BBC, Gabriela Chavez said she had been travelling between Seville and Madrid via train when the power went out and the AC and toilets stopped working.

“They have let everyone off the train but we have no idea how long we will have to wait for,” she told the BBC on Monday. “People at the top of the hill have flagged down cars who have dropped off water and crisps for us, which is kind [and] people from the village have come to drop off supplies for us.”

Flights were also impacted, with delays and cancellations at some airports. EasyJet said it was experiencing some disruption to operations at Lisbon and Madrid. It said the situation was “fluid” and told customers to check local advice.

As of Monday evening, aviation data company Cirium said 96 flights departing from Portugal – and 45 from Spain – had been cancelled.

Businesses have been severely affected. Some Ikea branches in Spain switched to backup generators and stopped customers from entering its stores.

The Madrid Open Tennis organisers have decided to cancel Monday’s event.

Elsewhere, Andorra and parts of France were also hit, but the Balearic and Canary Islands were not affected.