Gatwick’s runway has reopened after drones caused the airport to shut down for more than a day.
The airport said 765 flights were scheduled for departure and arrival.
Chief executive officer Chris Woodroofe told the BBC police have not yet found the operator of the drones. Police said it was possible they were an environmental activist.
He said extra “mitigating measures” from the government and military had given him “confidence to reopen”.
Thousands of passengers remain stranded at Gatwick as police continue their search for those responsible for the chaos, which started on Wednesday night.
Officers have so far failed to locate the drones or their pilot and had been considering plans to shoot a device down.
But Steve Barry, assistant chief constable at Sussex Police, said they were in a “much better position today”.
He told BBC Breakfast there were a “number of lines of inquiry” into the “very malicious and criminal behaviour”, including the possibility it could have been the work of an environmental activist.
Transport Secretary Chris Grayling said there was no evidence it was terror-related.
But he called it a type of disruption “we’ve not seen before” and “lessons need to be learned.”
He said the situation was “unprecedented, anywhere in the world”.
“Every possible measure will be put in place to make sure this can’t happen again”, he added.
Airport boss Mr Woodroofe would not be drawn on what measures would be taken if a drone was spotted again.
But he said: “Additional mitigating measures provided by government agencies and military have given me the confidence to reopen the airport.”
Gatwick said the flights planned for Friday would have about 126,000 passengers on board. About 140 flights have been cancelled.
Mr Woodroofe added: “My intention is to get those passengers to their destinations so that they can enjoy their Christmas.”
The first flights in and out of Gatwick were two Easyjet and two BA departures and one China Eastern Airlines arrival from Shanghai.
About 120,000 people had been due to fly since the runway closed.
Gatwick continues to advise passengers to check their flight status before turning up at the airport.
Passengers have complained of “freezing” temperatures in the south terminal, while others found themselves stuck abroad after inbound flights were either cancelled or diverted.
Dozens of passengers contacted the BBC to say uncertainty had led them to ditch their festive travel plans or spend extra money on new flights and hotel stays.
Budget airline Ryanair said it was switching all of its Gatwick flights to operate in and out of Stansted airport on Friday.