Local residents and animals evacuated shortly before lava and huge plume of smoke rise high into sky
Experts had said an eruption on the island, on the far west of the Atlantic archipelago, was not imminent.
However an intensification of earthquakes in recent hours resulted in an eruption at 3.15pm local time (3.15pm BST) no domingo.
Dramatic video showed a large plume of smoke rising into the sky from a volcano in the Cumbre Vieja national park, in the south of the island.
The Canary Islands Volcanology Institute, which had been monitoring the area, tweeted an image of the “eruptive column”.
Just before, authorities had evacuated about 40 people with mobility problems and farm animals from the villages around the volcano.
Soldiers were deployed to help with the evacuation, the defence ministry said, and it is expected that more residents will be evacuated from surrounding towns.
It followed after a 3.2-magnitude earthquake was recorded just 100 meters below the surface on Saturday, while several more tremors were detected on Sunday morning, the largest at a magnitude of 3.8.
No total, mais que 6,600 small earthquakes have been registered in the Cumbre Vieja area since the latest earthquake swarm activity began on 11 setembro.
On Sunday morning alone, the National Geographic Institute (IGN) disse 327 earthquakes had been detected.
Authorities decided on Monday to raise the threat level to yellow, requiring residents in at-risk zones to be prepared to evacuate.
The Scientific Committee of the Volcano Risk Prevention Plan said stronger earthquakes were “likely to be felt and may cause damage to buildings”.
The committee of experts also noted that a stretch of the island’s southwest coast was at risk from landslides and rock falls.
La Palma’s last eruption was in 1971, when one man was killed when he took photographs near the lava flows but no property was damaged.