By Sarah Young and Kate Holton

LONDON (Reuters) - Britain sent troops to its second-biggest airport after an unprecedented attempt to cripple Christmas travel with large drones forced all flights to be canceled on Thursday.

As thousands of passengers waited at Gatwick Airport, south of London, police hunted unsuccessfully for the operators of the large drones which reappeared near the airfield every time the airport tried to reopen the runway.

Police said there was no indication of a terrorism motive behind the devices, which first appeared on Wednesday night.

"The assessment earlier on today was that we wouldn't be using firearms," Detective Chief Superintendent Jason Tingley said late on Thursday. "This is continually reviewed so you will know and have seen that we have firearm officers deployed."

Europe's air traffic control agency Eurocontrol said the airport would remain closed until 0600 GMT on Friday.

The armed forces were also deployed to help resolve the situation.

"We are there to assist and do everything we can," Defence Minister Gavin Williamson told reporters.

Drones were seen as recently as the last hour, a Gatwick spokesman said at about 2200 GMT on Thursday, more than 24 hours after their first sighting.

The airport said flights would remain shut down for the rest of the evening on a day when 115,000 people were scheduled to pass through, many en route to seasonal breaks.

Prime Minister Theresa May's spokesman condemned the standoff as "irresponsible and completely unacceptable".

Passenger Ani Kochiashvili had been bound for Georgia but spent six hours overnight sitting on a plane with her children.

"I'm very annoyed because I'm with two kids, a three-month-old and three-year-old," she told Reuters by phone among thousands camped in the terminal.

"They require a lot of space and food and changing and all that, and the airport is crazy busy so it's challenging."

Flights were halted at 2103 GMT on Wednesday after two drones were spotted near the airfield, triggering the biggest disruption at Gatwick since a volcanic ash cloud in 2010.

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Police said more than 20 units were hunting the operators near Gatwick airport, 50 km (30 miles) south of London.

Transport minister Chris Grayling said it was clearly a deliberate act. "This is a commercial-sized drone," he said. "Every time Gatwick tries to re-open the runway, the drones reappear."

Grayling temporarily lifted night-flying restrictions at other airports to ease congestion caused by diverted aircraft, Sky News reported.

"COMPLETE MAYHEM"

With a surge in public enthusiasm for drones, there has been an increase in near-collisions by unmanned aircraft and commercial jets in recent years.

The number of near misses between private drones and aircraft in Britain more than tripled between 2015 and 2017, with 92 incidents recorded last year, according to the UK Airprox Board regulator.

Richard Parker, head of air traffic management technology firm Altitude Angel, said this was the first time a major airport had been hit by such a sustained and deliberate incursion into its airspace.

"It's sophisticated, not from a technology side, but it's organized. People have charged lots of batteries, and are deliberately trying to avoid being caught, probably by driving around to different locations," he told Reuters.

"It really is unprecedented."

Gatwick's Chief Operating Officer Chris Woodroofe described one of the drones as a heavy industrial model.

"The police advice is that it would be dangerous to seek to shoot the drone down because of what may happen to the stray bullets," he told BBC radio.

Drone expert Peter Lee of Portsmouth University said he and others had been anticipating disruption.

"One of my concerns about today is that it may well encourage copy-cat incidents because you can achieve a high amount of disruption for a very, very low cost," he said.

It is illegal to fly drones within 1 km (0.6 mile) of a British airport boundary, punishable by five years in prison.

Even after Gatwick re-opens, the backlog and disruption are expected to last for days.

Gatwick said it was working with its airlines, the biggest of which also include British Airways and Norwegian , on recovery plans once the runway re-opens.

Safety was its "foremost priority", it said.

Gatwick, which competes with Europe's busiest airport Heathrow, west of London, had previously said Sunday would be its busiest day of the festive period.

Passengers took to Twitter to share their stories.

One waiting at the airport said: "At Gatwick Airport, drone chaos, surprisingly good natured, but complete mayhem."

(Reporting by Sarah Young in London and Ismail Shakil in Bengaluru; Additional reporting by Alistair Smout and Andrew MacAskill; Editing by Andrew Cawthorne, Alison Williams and James Dalgleish)