LONDON — London Heathrow Airport said it was “fully operational” on Saturday, after an almost daylong closure sparked by an electrical substation fire. But airlines warned that severe disruption will last for days as they scramble to relocate planes and crews and get travelers to their destinations.

The airport’s boss said he was proud of Heathrow’s response to the incident. But inconvenienced passengers, angry airlines and concerned politicians sought answers about how one seemingly accidental fire could shut down Europe’s busiest air hub.

“We have hundreds of additional colleagues on hand in our terminals and we have added flights to today’s schedule to facilitate an extra 10,000 passengers traveling through the airport,” Heathrow said in a statement, advising passengers to check with their airline before going to the airport.

British Airways, Heathrow’s biggest airline, said it expects to operate about 85% of its 600 scheduled flights at the airport on Saturday. It said that “to recover an operation of our size after such a significant incident is extremely complex.”

More than 1,300 flights were canceled and some 200,000 people stranded Friday after an overnight fire at a substation 2 miles (3.2 kilometers) away from the airport cut power to Heathrow, and to more than 60,000 properties.

Residents in west London described hearing a large explosion and then seeing a fireball and clouds of smoke when the blaze ripped through the substation. The fire was brought under control after seven hours, but the airport was shut for almost 18. A handful of flights took off and landed late Friday.

Police said they do not consider the fire suspicious, and the London Fire Brigade said its investigation would focus on the electrical distribution equipment at the substation.

Still, the huge impact of the fire left authorities facing criticism that Britain’s creaking infrastructure is ill-prepared to deal with disasters or attacks.

The British government acknowledged that authorities had questions to answer and said a rigorous investigation was needed to make sure “this scale of disruption does not happen again.”

Heathrow chief executive Thomas Woldbye said he was “proud” of the way airport and airline staff had responded.

“Remember, the situation was not created at Heathrow Airport,” he told the BBC. “The airport didn’t shut for days. We shut for hours.”

He said Heathrow’s backup power supply, designed for emergencies, worked as expected, but it wasn’t enough to run the whole airport, which uses as much energy as a small city.

“That’s how most airports operate,” said Woldbye, who insisted “the same would happen in other airports” faced with a similar blaze.

Heathrow is one of the world’s busiest airports for international travel, and saw 83.9 million passengers last year.

Passengers on about 120 flights were in the air when the closure was announced found themselves landing in different cities, and even different countries.

Friday’s disruption was one of the most serious since the 2010 eruption of Iceland’s Eyjafjallajokull volcano, which spewed clouds of ash into the atmosphere and shut Europe’s airspace for days.

Mark Doherty and his wife were halfway across the Atlantic when the inflight map showed their flight from New York’s John F. Kennedy Airport to Heathrow was turning around.

“I was like, you’re joking,” Doherty said before the pilot told passengers they were heading back to New York.

Doherty called the situation “typical England — got no back-up plan for something happens like this. There’s no contingency plan.”