London’s Heathrow Airport closed on Friday after a fire at a nearby electrical substation caused a power outage, disrupting travel for tens of thousands of passengers planning to fly in or out of Europe’s busiest airport.

The airport was ready to resume flights as of Friday afternoon local time, the airport posted on X, and said it hopes to run a “full operation” on Saturday.

More than 800 flights were canceled in and out of the airport on Friday, according to flight-tracking site FlightAware, as of the most recent update, upending travel at the major hub and connecting airport.

Airlines warned travelers that disruptions could continue into the weekend, and the airport posted that travelers shouldn’t go to the airport unless advised to do so by their airline.

London’s Metropolitan Police said that while there was “no indication of foul play,” the counterterrorism division would now lead the investigation into the fire.

“Given the location of the substation and the impact this incident has had on critical national infrastructure, the Met’s Counter Terrorism Command is now leading enquiries,” the force said in a post on X.

“This is due to the specialist resources and capabilities within that command that can assist in progressing this investigation at pace to minimise disruption and identify the cause,” it said.

“Heathrow is experiencing a significant power outage across the airport. … Whilst fire crews are responding to the incident, we do not have clarity on when power may be reliably restored,” a Heathrow spokesperson said earlier Friday.

Airlines around the world due to operate flights into and out of Heathrow told passengers to stay home.

The fire and airport closure left thousands of travelers stranded. British Airways was the most affected airline, with over half of its Friday schedule canceled.

The airline said it would offer “flexible options” for rebooking to passengers set to travel to or from Heathrow on Friday through the weekend, in an online post.

“Our teams are currently working hard to review our long-haul schedule as well as the implications for our schedule tomorrow and beyond,” it said in a statement.

As the fire appears to be outside of the airlines’ control, they may not be required to cover compensation, according to a note issued by Citi on Friday.

American Airlines, a British Airways partner across the Atlantic, said almost 20 flights from Thursday were diverted or cancelled and that it provided overnight hotels for affected customers. It canceled another 20 on Friday.

It was not clear when its operations would resume, and a spokeswoman said it would restart Heathrow operations “when airport conditions allow.”

European travel and leisure stocks fell on news of the airport closure.

Ed Miliband, U.K. energy minister, described the fire as “catastrophic,” according to Reuters, adding that the airport’s backup generator had been affected by the blaze.

Speaking to ITV’s “Good Morning Britain,” Miliband said the National Grid told him “it’s like a fire they’ve never, kind of, quite seen anything like the scale of what happened before,” according to a post by the program on X.

Miliband added that the National Grid was trying to use another backup system to restore power to the airport.

Power cuts also affected about 16,000 homes around the airport. As of 8 a.m. GMT, electrical supply was restored to all but around 4,900, according to the U.K. energy company Scottish and Southern Electricity Networks.

Heathrow Airport has an estimated 1,300 takeoffs and landings at the airport per day, according to its website. It handled a record 83.9 million passengers last year — a nearly 6% increase from 2023.

Speaking to “Good Morning Britain,” Miliband said on Friday, “We’ve got to understand why this happened, and we’ve got to work out what the lessons are for the resilience of our infrastructure.”

He said the National Grid is looking at whether there is “sufficient resilience” in place at the airport, given that the fire also affected a backup generator.

“It makes Heathrow look quite vulnerable. And therefore, we’ve got to learn lessons … about not just Heathrow, but how we protect our major infrastructure,” Miliband said.

Willie Walsh, CEO of the International Air Transport Association, or IATA, an airline industry group, criticized Heathrow Airport for being “totally dependent on a single power source without an alternative,” in an online statement, describing it as a “total planning failure” by the airport.

Walsh questioned who would cover the costs of the resulting travel disruptions.

“We must find a fairer allocation of passenger care costs than airlines alone picking up the tab when infrastructure fails,” he said. “Until that happens, Heathrow has very little incentive to improve.”

“What we also need to take into account is over and above passenger traffic, over 4,000 tons of cargo go through Heathrow every single day,” she told CNBC’s “Squawk Box Europe.”

More than 1.4 million tonnes of cargo flew in and out of Heathrow in 2023, according to a post on the airport’s website, with 90% of goods transported in the hold of passenger aircraft.

Airport officials said they will update travelers “when more information on the resumption of operations is available.”

Travelers can check Heathrow Airport’s website or social media platforms, including X, for the latest information.