The seismic crisis began at 05:15 GMT with a tremor that indicated the arrival of magma close to the surface.
"The Piton de la Fournaise volcano's eruption began at 03:30 a.m. local time [07:30 GMT] on December 22, 2021 on the southern slope", the observatory said.
Earlier in the day, the observatory's team observed at least 3 eruptive cracks open on the southern slope of the volcano. Public access, as well as the landing of aircraft in the area of the volcano, are now prohibited.
The volcano's last eruption took place on 9 April 2021. Piton de la Fournaise is one of the most active volcanoes on the planet. It first awoke 500,000 years ago and has been erupting twice a year on average for the past two decades. It usually emits fountains of lava that flow down its flanks.