The Italian Institute of Geophysics and Volcanology said the volcano came to life at about 4 pm (15:00 GMT), sending an ash plume about a kilometer into the air.
A fountain of lava towered above Etna for about an hour before finally subsiding at 7 pm, but streams of lava continued flowing down the western and southeastern slopes.
The Volcanic Ash Advisory Center in Toulouse warned that the gray ash cloud rose up to more than 10,000 meters.
Ash and pumice fell on several villages huddled around the volcano, reaching as far as the town of Catania, which closed the international airport and grounded flights. There have been no reports of damage.