But what some thought to be a close encounter of the third kind turned out to be a string of some 60 satellites
launched by US-based SpaceX hours earlier as part of its "Starlink" constellation.
The row of satellites which are part of a plan by billionaire Elon Musk's firm to provide internet from space, glided
across Dutch skies around 1:00 am (2300 GMT).
Shortly afterwards, Dutch website www.ufomeldpunt.nl was inundated with more than 150 sighting reports, with
astonished spotters describing a "bizarre train of stars or lights moving across the skies at constant speed".
"There's a long line of lights. Faster than a plane. Huh?" one spotter reported, while another called it a "star
caravan" and one saying "I have it on film".
One spotter simply texted: "WTF?"
"I didn't know what to make of it," an unnamed witness later told the NOS public broadcaster.
"Is it Russia attacking the US? Are they UFOs? Seriously, I didn't know," the witness said.
One of SpaceX's Falcon 9 rockets blasted off without incident from Cape Canaveral in Florida at around 0230 GMT on
An hour after liftoff, the rocket began to release the satellites at an altitude of 280 miles (450 kilometres).
The satellites then had to separate and use their thrusters to take up their positions in a relatively low orbit of 340
miles (550 kilometres).
Each of the satellites weighs 227 kilograms (500 pounds) and was built in-house in Redmond, near Seattle.
Starlink will become operational once 800 satellites have been activated, which will require a dozen more launches.
One Dutchman who remained unfazed was satellite spotter Marco Langbroek, who knew what the mysterious lights
were -- and had his camera on hand.
"I cheered them on, the moment they appeared," he told the NOS.