Activity inside Mount Merapi intensified on Monday morning as the Indonesian volcano spewed clouds of hot ash, rock fragments and gas.

An avalanche of debris and ash rolled down the volcano's western flank, travelling nearly four kilometres, said the region's chief vulcanologist, Ratdomopurbo. Large explosions in the crater followed.

Mount Merapi releases a huge cloud of ash early Monday. Scientists say the volcano's activity has intensified and an eruption is almost certain.
(Bayu Pamungkas/Associated Press)

Indonesians watched the 3,000-metre volcano's activity on live television over the weekend.

Authorities ordered thousands of people living on or near the mountain, which is located on the densely populated island of Java, to leave but many refused. On Monday, some of those were reconsidering.

In villages near the peak, only a few young men remained as police toured the danger zone urging the holdouts to leave. Others had been seen leaving the area in public minivans or trucks.

The holdouts are mostly peasant farmers intent on staying behind to protect their land and livestock.

The mountain is shrouded in mystic beliefs. Holy men held a ceremony on Sunday they hoped would ward off an eruption. They burned incense and floated offerings of food in a river that runs down the volcano's slopes.

After monitoring weeks of activity inside the volcano, scientists raised the alert on Saturday.

More than 4,500 people living near the crater or along the banks of rivers that could become streams of hot lava were moved to safer areas where they have taken shelter in mosques, government buildings and schools.

Merapi last erupted in 1994, killing 60 people when it belched a searing gas cloud.

Indonesia has 129 active volcanoes.