"As a precautionary measure, we have stopped departure while we investigate".
Police will also be allowed to fine users up to Â£100 (112 euros, $128) for failing to comply when instructed to land a drone, or not showing registration to operate a drone.
London's Metropolitan Police said it received a report about a drone "in the vicinity" of the airport.
Users took to Twitter to share pictures live from the scene.More news: Buccaneers hire Arians as head coach
Passengers were told their flights would not be leaving as police helicopters were scrambled to the airfield.
After the drones are detected, staff at Gatwick airport can then use an inhibitor to jam the signals - or radio frequencies - used by the drone, bringing it safely down to earth.
While that would boost punctuality, "flight paths could overfly areas that are not affected by Heathrow arrivals today", the airport said in the study on proposed airspace changes.
A drone can cause serious damage to an aircraft.
Heathrow wants to accommodate an extra 25,000 flights per year ahead of the planned opening of a new runway at the airport.More news: The San Antonio Spurs decimated the Toronto Raptors in Leonard’s return
Heathrow has said it was working closely with authorities including the police and looking at relevant technology to combat the threat of drones.
Travel experts estimate that around 40 flights were delayed in the shutdown, and passengers tweeted their frustration at chaos paralysing yet another United Kingdom airport.
The exclusion zone around airports will be extended to approximately a 5km-radius (3.1 miles), with additional extensions from runway ends.
Coincidentally, this comes on the same day the United Kingdom government's Department for Transport announced a series of new drone-related powers for police, including new airport exclusion zones.More news: National Football League world reacts to Cody Parkey's missed field goal