The Santiaguito volcano showered sand and ash on Monday over a large area of western Guatemala in an "unusual" and "violent" eruption, the national seismological institute said.
Winds were carrying the ash in a northeasterly direction from the volcano, 2,500 metres above sea level in the province of Quetzaltenango, 206 kilometres west of Guatemala City, capital of Guatemala, it said.
The ash plume spread across six provinces, raising fears of damage to crops, the institute said, while classes were suspended at schools in three provinces.
The National Disaster Reduction Coordination Agency, CONRED, increased its alert level to orange, one level below the maximum red, its director Alejandro Maldonado said.
The volcanic activity propelled grey ash more than 8,300 metres into the air, and CONRED advised civil aviation authorities to ban flying within 15 kilometres of the volcano.
Santiaguito's worst eruption occurred in 1929 when 2,500 people were killed.