Airlines have been issued a "red warning" about the danger of volcanic ash in the skies close to Bali after Mount Agung emitted a thick plume of smoke reaching 4,000m (13,123 feet).
It is the second major emission from the Indonesian island volcano this week, and flights have been disrupted.
The red warning means an eruption is forecast to be imminent, with significant emission of ash likely.
Authorities have begun distributing masks in areas amid ash fall.
Bali is a major tourist destination, although the main destinations of Kuta and Seminyak are about 70km (43 miles) from the volcano.
The island's main airport is for now operating normally, but some airlines have cancelled flights over fears as volcanic ash can damage plane engines.
Hundreds of thousands of masks have been prepared by Indonesia's Disaster Agency
An airport official told the AFP news agency that dozens of flights had been affected.
"Tourism in Bali is still safe, except in the danger (zone) around Mount Agung," Indonesia's Disaster Mitigation Agency said in a statement.
It told people within a 7.5km exclusion zone to "immediately evacuate" in an "orderly and calm manner".
Passengers are pictured waiting at Ngurah Rai airport on Saturday
About 25,000 people are thought to still be in temporary shelters after more than 140,000 people fled earlier this year after increased volcanic activity prompted fears a major eruption was imminent.
Most of the islanders outside of the immediate exclusion zone were ordered to return home at the end of September , and the mountain has been intermittently rumbling since.
According to official estimates, the holiday island lost at least $110m (£83m) in tourism and productivity during the major evacuation.
Indonesia sits on the Pacific "Ring of Fire" where tectonic plates collide, causing frequent seismic and volcanic activity.
It is home to more than 130 active volcanoes. The last time Mount Agung erupted, in 1963, more than 1,000 people died.