The volcano overlooks the tourist city of Antigua and is one of Central America's most-active volcanoes.
Guatemala's National Seismology, Vulcanology, Meteorology and Hydrology Institute said in a statement Saturday that Fuego shot ash 16,400 feet (5,000 meters) into the air. Lava flows reached up to 3,280 feet (1,000 meters) in length.
David de Leon, spokesman for the national disaster agency, said authorities have raised the alert level because Fuego is in an effusive stage. The measure involves closing nearby highways, installing monitoring stations and readying emergency workers.
De Leon said while no evacuations have been ordered, there might be if Fuego's activity increases.