London (CNN) The United Kingdom's second-biggest airport has been closed on one of the year's busiest days for travel after drone sightings, which police have described as a "deliberate act."

London's Gatwick Airport has been closed since 9 p.m. Wednesday -- bar a brief reprieve of 45 minutes early Thursday -- after drones were spotted near the airfield.

Passengers due to fly on Thursday evening or Friday were being told not to go to the airport without first checking the status of the flight with the appropriate airline.

Police were still on the hunt for the drone operators who have brought the airport to a standstill, causing travel chaos for hundreds of thousands of passengers just days before Christmas.

"Each time we believe we get close to the operator, the drone disappears; when we look to reopen the airfield, the drone reappears," Sussex Police Superintendent Justin Burtenshaw told the UK's Press Association.

A police helicopter flies over the runway at Gatwick airport.

The Ministry of Defence said that it had deployed specialist equipment to assist Sussex Police in their efforts.

The most recent drone sighting was at about 4:30 p.m. ET (9:30 p.m. local time), according to an airport spokesman. Airport Chief Operating Officer Chris Woodroofe could not say when the major international airport, located south of London, would reopen.

The chief executive officer of the airport, Stewart Wingate, said he hoped passengers would appreciate that officials were prioritizing their safety.

"I would like to repeat how sorry we are for the inconvenience this criminal behaviour has caused passengers and we share their real anger and frustration that it has happened," he said.

Aviation expert Jon Parker told CNN he'd "seen nothing on this scale before," in terms of deliberate disruption by a drone to a major UK airport.

"The usual practice (when a drone is spotted) is to suspend flights for half-an-hour, which is the usual battery lifespan for drones," explained Parker, a former Royal Air Force fighter pilot and head of drone training company Flyby Technology.

But in the case of Gatwick, "whoever is responsible for this has had several batteries and have brought their drones back to the ground to put new batteries on them," he said.

It is illegal to fly drones within 1 kilometer (0.6 miles) of a UK airfield boundary, with perpetrators facing up to five years in prison.

There is no indication the incident at Gatwick is terror-related, Sussex Police said. It added that 20 police units from two forces were searching for the drone operators.

People wait near the departures gate at Gatwick airport on Thursday.

A spokesperson for Prime Minister Theresa May called the act "irresponsible and completely unacceptable."

The government said it would lift night-time flight restrictions at other London airports to help ease disruption caused by Gatwick's closure.

"It's right and proper we try and sort people's Christmases out," Transport Secretary Chris Grayling told the BBC.

'Industrial specification'

The drones used to disrupt flights at Gatwick are believed to be of an "industrial specification," according to Sussex Police.

This means that a device is not an off-the-shelf consumer drone, but something bigger or more complex, or perhaps expertly home-made, said Lewis Whyld, CNN's drone operator and photojournalist.

He added that drones can have huge ranges, with some of the more powerful ones controlled from up to 10 miles (16 kilometers) away.

Drones don't necessarily need an operator, added Whyld. They could be pre-programmed to follow a route using GPS, he said.

Passengers left in the lurch

Gatwick was thrown into turmoil on Wednesday and Thursday, with passengers stranded at the terminal or re-routed to other airports.

About 110,000 passengers on 760 flights were scheduled to depart and land at Gatwick on Thursday, a spokesperson told CNN.

Passengers due to land at Gatwick were instead arriving in Manchester, Luton or Heathrow in the UK, or even as far afield as Paris and Amsterdam.

Passengers wait for announcements at Gatwick South Terminal.

One passenger, Matt, said on Twitter he was waiting to board a coach at Manchester in the north of England -- two hours after he was supposed to arrive at Gatwick at the other end of the country.

Ryanair tweeted that flights scheduled for Friday would operate to and from Stansted Airport.

At Berlin's Tegel Airport, heavily pregnant British woman Imogen Fletcher was left stranded for hours after her EasyJet flight to Gatwick was canceled. EasyJet canceled all flights to and from Gatwick airport.

The IT developer told CNN she had been on her way home for Christmas with her husband and daughter, but on Thursday morning she was left scrambling to find another flight.

Passenger John Belo told CNN he was stuck on a plane at Gatwick for four hours. Belo had been due to fly to Porto in Portugal at 8.45 p.m. on Wednesday night, in what should have been a two-and-a-half hour journey. But after hours of delays he finally arrived in the city at 2 a.m. the following morning.

The shutdown has sparked delays and diversions in the run-up to the Christmas getaway.

Chaotic scenes at Gatwick

Passengers stranded at Gatwick described "total chaos" inside the terminal, with flights suspended and little information from staff.

Eddie Boyes, who was due to fly to Odessa in Ukraine, was stuck at the terminal with his wife and 4-year-old son for nine hours. He told CNN they had received "no information from the airport -- any information I have managed to get has been from social media.

"We have been given food vouchers totaling £30 ($38), offered a hotel room initially but very shortly afterwards this was retracted."

"People are sleeping on the floor in south terminal," he said, adding it was an "utter shambles."

Stranded passengers sleeping on the floor.

Gatwick doesn't usually operate overnight, but morning is its busiest time, an airport spokesperson told CNN. The spokesperson warned passengers not to make their way to the airfield without checking on the status of their flight first.

"We 're sorry for the inconvenience today, but the safety of our passengers and staff is our no.1 priority," the airport said in a statement.

Drone dangers

Drones flying too close to commercial flights pose a serious threat to larger aircraft, and can be sucked into engines or crash into the cockpit window, injuring or killing a pilot.

It is illegal to fly a drone higher than 400 feet (120 meters), or within 150 feet (50 meters) of people and properties.

Drone operators found to have endangered the safety of an aircraft could also face up to five years in prison.

CORRECTION: This story has been updated with the correct time frame for the number of flights landing and departing Gatwick on Thursday.