UK police say they have made two arrests in connection with criminal drone activity at Gatwick Airport, and urged the public and passengers around the airport to remain vigilant.

Key points: Sussex Police say investigations still ongoing after two arrests

Sussex Police say investigations still ongoing after two arrests Gatwick had to briefly close for another drone sighting only 11 hours after reopening

Gatwick had to briefly close for another drone sighting only 11 hours after reopening Police say there is a "whole spectrum" of possible motivations for closing Britain's second-busiest airport

Britain's second-largest airport reopened on Friday after a mystery saboteur wrought 36 hours of travel chaos for more than 100,000 Christmas travellers by using drones to play cat-and-mouse with police snipers and the army.

"Our investigations are still ongoing, and our activities at the airport continue to build resilience to detect and mitigate further incursions from drones, by deploying a range of tactics," Sussex Police said in a statement.

"We continue to urge the public, passengers and the wider community around Gatwick to be vigilant and support us by contacting us immediately if they believe they have any information that can help us in bringing those responsible to justice."

Britain deployed the army, police snipers and unidentified military technology to guard the airport against what transport minister Chris Grayling said were thought to be several drones.

"This kind of incident is unprecedented anywhere in the world," he said.

The defence ministry refused to comment on what technology was deployed, but drone experts said airports needed to deploy specialist radar reinforced by thermal imaging technology to detect such unmanned flying vehicles.

Stranded passengers have described freezing conditions while sleeping on the floor and chairs. ( Reuters: Toby Melville )

The Telegraph newspaper reported earlier that the perpetrator had circled the drone around the airport control tower and flashed its lights.

A description of the drone by witnesses had enabled experts to determine the model of the machine, according to the report.

Despite the measures, another drone sighting caused the airport to close again just 11 hours after reopening.

Incoming planes circled over Gatwick because they could not land and outgoing planes were kept on the tarmac while the drone report was investigated.

Gatwick Airport later confirmed the runway had been reopened after less than an hour. A spokeswoman said the brief second shutdown was precautionary, and new measures had been put in place to ensure the safety of travellers.

The airport had been shut down all day on Thursday and part of Wednesday night (local time) when authorities said drones repeatedly violated the airport perimeter, threatening the safety of incoming and outgoing planes.

Sorry, this video has expired Travellers describe chaos at Gatwick airport

The motive for the drone invasion was not clear, but British police said there were no indications it was "terror related".

The Thursday shutdown at Gatwick threw tens of thousands of passengers into massive travel chaos, since about 110,000 people had been scheduled to pass through the airport that day, one of the busiest travel days of the year.

The figure for passengers expected for Friday was even higher.

Gatwick serves more than 43 million passengers a year.

The hundreds of travellers stuck overnight at Gatwick by Thursday's closure described freezing conditions as they slept on benches or the airport floor. Many complained they were not being kept informed about re-routed flights.

About 145 of the 837 flights at Gatwick on Friday had already been cancelled before the latest reported drone sighting, the airport said.

In the afternoon, it strongly warned passengers to "check the status of your flight with your airline before departing for the airport".

World's airports now have to analyse their vulnerabilities

Counter-drone equipment was deployed on Gatwick Airport's roof. ( AP: John Stillwell )

The motivation of the drone operator, or operators, was unclear. Police said there was nothing to suggest the crippling of one of Europe's busiest airports was a terrorist attack.

Gatwick's drone nightmare is thought to be the most disruptive yet at a major airport and indicates a new vulnerability that will be scrutinised by security forces and airport operators across the world.

The army and police snipers were called in to hunt down the drones, thought to be industrial-style craft, which flew near the airport every time authorities tried to reopen it on Thursday.

No group has claimed responsibility publicly and police said there was no evidence another state was involved.

Sussex Police Assistant Chief Constable Steve Barry said they were keeping an open mind about who was responsible.

"In terms of the motivation, there's a whole spectrum of possibilities, from the really high-end criminal behaviour that we've seen, all the way down to potentially, just individuals trying to be malicious, trying to disrupt the airport," he said.

After a boom in sales, unmanned aerial vehicles have become a growing menace at airports across the world.

In Britain, the number of near misses between private drones and aircraft more than tripled between 2015 and 2017, with 92 incidents recorded last year.

Flights have resumed at Gatwick Airport following a cautionary closure. ( AP: Kirsty Wigglesworth )

ABC/wires