Authorities set up a shelter at an elementary school in Santa Lucia, the town closest to the volcano, and by Thursday night people had started trickling in. Most were women and children carrying blankets and going into bare classrooms.
Miriam Carumaco, 28, arrived to the shelter along with 16 members of her family.
"We heard loud thunder and then it got dark and ash began falling," Carumaco said. "It sounded like a pressure cooker that wouldn't stop."
Carumaco said parents sent their children to school despite the darkening skies, but that classes were later cancelled and teachers walked them home.
The emergency agency said lava rolled nearly 2,000 feet down slopes billowing with ash around the Volcan del Fuego, a 12,346-foot-high volcano whose name translates as "Volcano of Fire."
"A paroxysm of an eruption is taking place, a great volcanic eruption, with strong explosions and columns of ash," said Gustavo Chicna, a volcanologist with the National Institute of Seismology, Vulcanology, Meteorology and Hydrology. He said cinders spewing from the volcano were settling a half-inch thick in some places.
He said extremely hot gases were also rolling down the sides of the volcano, which was almost entirely wreathed in ash and smoke. The emergency agency warned that flights through the area could be affected.
There was a red alert, the highest level, south and southeast of the mountain, where, Chicna said, "it's almost in total darkness."
He said ash was landing as far as 50 miles south of the volcano.
By Thursday evening, the ash plume had decreased to a little more than a mile high, partly due to rain, which diminished the potential risk to aviation, said Jorge Giron, a government volcanologist. He said ash continued to fall heavily, however, and advised residents near the volcano but outside of evacuation zones to clean their water systems before using them, and to not leave their homes because of the ash.
He said a red alert would be in effect until 4 a.m. local time.
Teresa Marroquin, disaster coordinator for the Guatemalan Red Cross, said the organization had set up 10 emergency shelters and was sending hygiene kits and water.
"There are lots of respiratory problems and eye problems," she said.
Many of those near the volcano are indigenous Kakchikeles people who live in relatively poor and isolated communities, and authorities said they expected to encounter difficulties in evacuating all the affected people from the area.
Officials in the Mexican state of Chiapas, on the border with Guatemala, said they were monitoring the situation in case winds drove ash toward Mexico.