LONDON — Some flights resumed at Britain's second-busiest airport Friday after drones shut it down for 36 hours, stranding thousands of passengers just days before Christmas.

Gatwick Airport confirmed that its runway re-opened at 6 a.m. local time (1 a.m. ET), but departures and arrivals remained restricted. It warned passengers to expect delays and cancellations again on Friday.

Scene inside #Gatwick South Terminal check-in Hall this morning as a limited service gets underway: @euronews pic.twitter.com/u4oNzgeR6Q — Vincent McAviney (@VinnyMcAv) December 21, 2018

The prospect of a plane colliding with what police described as industrial-grade drones forced authorities to ground all flights in and out of Gatwick on Thursday.

Hundreds slept on benches and floors at the airport waiting for updates.

“It has ruined the holiday for the whole of our family,” frustrated passenger Anne Cracknell told NBC News late Thursday. “We've saved up all year to get this holiday and it’s just been shattered, all for somebody flying drones.”

The drones were first spotted Wednesday night near the airport, which is located about 28 miles south of central London.

It's not clear who was flying the drones, but police say there is no suggestion that terrorism is involved.

Passengers wait for information on delayed flights at Gatwick Airport on Thursday. Facundo Arribalaga / EPA

Chris Grayling, the U.K.'s transport secretary, told the BBC that drones appeared to be deliberately disrupting air traffic.

"Every time Gatwick tries to re-open the runway, the drones reappear," he said.

Grayling warned the perpetrators could face prison sentences of up to five years.

Airport officials said government agencies and the military worked overnight to put "measures in place" that reassured authorities the runway could be re-opened safely.

Sussex Police Assistant Chief Constable Steve Barry said the size of the drones made the potential option of shooting them down a difficult task.

“If anyone has seen the video footage of the drone in the air previously, you will know that it’s a very small dot in the sky,” he said.

However, Barry said that shooting them remained a possible tactic if they appeared again.

Gatwick Airport Chief Operating Officer Chris Woodroofe said there were around 100 flight cancellations Friday morning. He told the BBC the disruptions could continue into the weekend.

With more flights being delayed, it’s getting busier and busier here at Gatwick. #GatwickDrone #GatwickAirport pic.twitter.com/bQCXf2IZiC — Anastasia Siapka (@AnastasiaSiap) December 21, 2018

More than 43 million passengers a year use Gatwick Airport. At least 100,000 had been scheduled to pass through on Thursday, one of the busiest travel days of the year.

The chaos left thousands of travelers scrambling to find alternative flights at the last minute.

“I’m very tired and very stressed out, it’s almost Christmas,” said Cherie Myrvang, who was among those stranded.

Stewart Wingate, the airport's chief executive officer, apologized for the flight delays and cancellations.

He blamed "criminal behavior," adding that the drones had aimed to “close the airport and bring maximum disruption in the run-up to Christmas.”