The stratovolcano southwest of the city of Antigua began erupting on Sunday, with ash and lava spewing from its crater.
It comes five months after almost 200 people were buried by volcanic ash and mud during a violent eruption in June. That eruption generated pyroclastic flows - fast-moving mixtures of very hot gas and volcanic matter - which descended down the slopes, engulfing communities such as El Rodeo and San Miguel Los Lotes.
Volcanologists say lava is rising 500m above the volcano's crater.
The ash cloud exceeds a height of 1km (3,280ft) above the 3,763m-tall (12,250ft) volcano.
Fuego is one of Latin America's most active volcanoes.