- One of only a handful of times mayflies have swarmed there in 40 years
- Efforts to clean up the river have helped the insects return to the area
- Spectacle become unlikely tourist attraction with scores watching
Passers-by were hit with blizzard-like conditions and the air was thick with the winged insects.
There were so many insects in the northern town of Tahitotfalu, about 18 miles from Budapest, that cyclists had to cover their faces and cars were barely visible, even with their headlights on.
These rare insects are one of the only mayfly species to swarm at night.
Like the more famous mayfly spectacle on the River Tisza, the swarming insects on the Danube have become an unlikely tourist attraction.
Scores of people crowded the pavements and bridges in the area to watch the intense swarm,
known as Dunavirág.
People watched from the roadside as the mayflies were attracted to the street lighting.
The mayflies have returned to the Danube after efforts to clean the water in the past couple of years.
The insects hatched for the first time in four decades last year and experts say a reduction in nitrate and phosphate levels.
It has been helped by the improvement to the sewage treatment plant in Vienna, further along the Danube.
The white-winged mayflies are about one centimetre in size and they have just a few hours to mate after hatching before they die.
The morning following the Dunavirág sees the ground littered with the yellow abdomens of male mayflies - the rest having disappeared overnight.
The swarm is not yet as famous as the River Tisza fly 'blooming season' that occurs from late spring to early summer each year.
The river becomes infested with millions of mayflies taking to the air to frantically find a mate before they die.
It reminds me of the old C&W song, "The girls all get prettier at closing time."