Officials stopped short of ordering evacuations following the so-called strombolian eruption, explosions that consist of relatively mild blasts but eject burning cinders and lava bombs.
One of the Central American country's most active volcanoes, Pacaya began to expel lava at dawn, creating an immense column of smoke, video footage from the institute showed.
"The Pacaya volcano has a very high level of activity, which has generated a strombolian type eruption, with strong explosions," the meteorological institute, Insivumeh, said in a statement.
David de Leon, spokesman for the National Coordinator for Disaster Reduction (Conred), said civil protection teams were checking several facilities that could be used as shelters as lava continued to flow late on Wednesday.
Insivumeh said five towns, home to around 3,000 people, were at risk from burning projectiles from the volcano, which is located 28 kilometers (17.4 miles) south of Guatemala City.
Pacaya experienced some minor activity for a month before erupting on Wednesday.
(Reporting by Sofia Menchu in Guatemala City, writing by Stefanie Eschenbacher; editing by Jane Wardell)
Reuters
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