The Bali volcano sent a massive ash cloud 4,000m (13,100 feet) into the sky after months of seismic activity.
Mount Agung spat powerful mudflows containing rocks and volcanic debris into the air and has caused chaos for airlines having to cancel hundreds of flights.
Expert for the Volcano disaster mitigation agency, Gede Suantika, has warned about the possibility of another huge eruption.
Speaking on BBC News, he said: “The volcano has entered the magmatic eruption phase.
“There is now the possibility of a strong explosive eruption.”
Airport spokesman Air Ahsanurrohim said 445 flights were cancelled, stranding about 59,000 travellers.
The ash cloud caused the closure of Bali’s main airport after the Volcano alert was raised to its highest level.
The National Disaster Mitigation Agency (NDMA) said Bali’s airport had closed for 24 hours and authorities would discuss reopening it on Tuesday after evaluating the situation.
Indonesia’s disaster mitigation agency said 40,000 people had been evacuated from near Bali’s erupting Mount Agung volcano, but tens of thousands still needed to move.
The agency also said that Bali was “still safe” for tourists except for the 4.7-mile zone around Mount Agung.
Australia’s Bureau of Meteorology said in an advisory from it’s Volcanic Ash Advisory Centre (VACC) in Darwin the eruption was “expected to be ongoing”.
Mount Agung’s last eruption, in 1963, left more than 1,000 people dead and destroyed several villages promoting fears about another significant eruption.
Some Bali residents have however refused to evacuate the area, according to a spokesman for Bali’s Disaster Management Agency.
Sutopo Purwo Nugroho said: “Some people within a radius of 8-10km from the summit crater of Mount Agung have not been displaced.
“They feel safe because the area was not damaged during the 1963 eruption. They will evacuate if it is dangerous.”
The British Government has urged nationals in Bali to heed the advice of authorities as Mount Agung’s volcanic activity could “escalate” at any time.
“Ash clouds have caused airport closure for periods in both Bali and Lombok and disruption to flights in the region,” the Government’s travel advice webpage said.