Storm Poly packed howling winds of up to 146 km/h (90 mph), toppling trees and forcing the cancelation of 400 flights from Amsterdam's Schiphol airport, one of Europe's busiest hubs.

Meteorologists said the storm was the strongest on record to hit the Netherlands in the summer months and issued a rare "code red" warning for millions of people in the low-lying nation to stay indoors.

A 51-year-old woman was killed when a tree fell on her car in the Dutch city of Haarlem, while a 64-year-old woman died after being struck by a falling tree in the German town of Rhede near the Dutch border, police said.

Two men were seriously injured in Amsterdam, one when a tree fell on his car, while a second was believed to have been hit by falling power lines, local media said.

A tree also fell on a houseboat in one of Amsterdam's historic canals, while another toppled onto a tram in The Hague, though no one was hurt.

Howling winds

Schiphol Airport said the number of flights would "gradually improve" as winds started to drop but would remain disrupted for the rest of the day.