“The toll was 25 dead as of 9:00 pm (0300 GMT Monday),” the spokesman for the National Coordinator for Disaster Reduction (Conred) said in a WhatsApp group.
Search and rescue operations for the missing and dead have been suspended due to low light and dangerous conditions, and will resume early on Monday morning, the spokesman said.
The eruption sent ash billowing over the surrounding area, turning plants and trees gray and blanketing streets, cars and people.
Farmers covered in ash fled for their lives as civil defense staffers tried to relocate them to shelters during the event.
Earlier, Conred chief Sergio Cabanas and President Jimmy Morales held a news conference, saying that the eruption left seven dead, 20 injured and affected more than 1.7 million people.
Morales announced a red alert for Escuintla, Chimaltenango and Sacatepequez, the areas most affected by the eruption, and an orange alert throughout the country.
The president said he and his government would determine whether to ask Congress to declare a state of emergency in the areas, while at the same time appealing to the population for calm.
Hundreds of personnel from the police, Red Cross and military have been dispatched to support emergency operations, Morales said.
Cabanas said that the dead included a civil protection official and others trapped by muddy material that descended from the 3,763-meter (12,346-foot) volcano.
Twenty people suffered burn injuries, and more than 3,000 were evacuated due to the eruption, which affected rural communities around the volcano as well as Antigua Guatemala, a colonial-era town very popular with tourists in the Central American country, he said.
There are also “missing persons, but we do not know how many,” Cabanas said, adding that lava had blocked entry to several communities.
Dense ash blasted out by the volcano shut down Guatemala City’s international airport, civil aviation said.
People were working to clean ash off the runways to get the airport operating again.
It is the second major eruption this year from the peak, following another that subsided at the beginning of February after sending ash towering 1.7 kilometers into the sky.
Guatemala has two other active volcanoes, Santiaguito in the west and Pacaya just south of the capital.