This is feeding demand for increasingly advanced technology to bring down or disable unwanted drones.

At one end of the scale, the Dutch national police recently bought several birds of prey from a start-up called Guard From Above to pluck unwanted drones from the sky, its CEO and founder Sjoerd Hoogendoorn said in an email.

Other approaches focus on netting drones, either via bigger drones or by guns firing a net and a parachute via compressed gas.

Some, like Germany's DeDrone, take a less intrusive approach by using a combination of sensors - camera, acoustic, Wi-Fi signal detectors and radio frequency (RF) scanners - to passively monitor drones within designated areas.

Newer start-ups, however, are focusing on cracking the radio wireless protocols used to control a drone's direction and payload to then take it over and block its video transmission.

Singapore's TeleRadio Engineering uses RF signals in its SkyDroner device to track and control drones and a video feed to confirm targets visually.

DroneVision Inc of Taiwan, meanwhile, says it is the first to anticipate the frequency hopping many drones use. Founder Kason Shih says his anti-drone gun - resembling a rifle with two oversized barrels, coupled with a backpack - blocks the drone's GPS signals and video transmission, forcing it back to where it took off via the drone's own failsafe features.