Villagers burned incense and floated offerings to the spirits, hoping to ward off an eruption of Mount Merapi, but activity at the volcano intensified on Monday — with one blast sending ash, rock and gases more than two miles down the slope.

A scientist warned on Sunday that a growing lava dome could collapse. On Monday, as activity increased, villagers who had not left were told to stand by for possible evacuation and waited in groups by the side of the road on the slopes of the volcano.

One of the eruptions was the most powerful yet, sending ash, rock fragments and volcanic gas almost 2 1/2 miles down the mountain's western flank, said Ratdomopurbo, the region's chief vulcanologist.

Despite a government evacuation order, many farmers were in the fields to tend animals and crops on the volcano's fertile slopes, ignoring black clouds billowing into the sky and fresh scars scorched by lava flows on the mountain's western flank.

"I cannot force them," said Widi Sutikno, the official coordinating the government's emergency operation. "All I can do is tell them to keep looking up at the mountain and have a motorbike ready."

More than 4,500 people living in villages closest to the crater or next to rivers that could provide paths for hot lava had been evacuated by Sunday, a day after scientists raised the alert status for Merapi to the highest warning after weeks of volcanic activity.

Sutikno said 18,000 others who live lower down the slopes were not considered in immediate danger and had not been ordered to leave their homes on the 9,800-foot mountain that rises from the plains of Indonesia's densely populated Java Island.

In one of the villages in the shadow of Merapi, holy men and hundreds of people lit incense and set rice, fruit and vegetables floating down a river in a ceremony they believed would appease the spirits and prevent an eruption.

"It's bound to help," Parsi, a villager who like many Indonesians using only one name, said after the ceremony. "Everyone around here believes in this. It is in our blood."

Although most Indonesians are Muslim, many also worship ancient spirits, especially in Central Java province.

"All the things we are doing here are to try to make us safe," said Assize Asyhori, an Islamic preacher who took part in the ceremony. "Only Allah knows if Merapi will explode."

Police at roadblocks prevented vehicles from getting within five miles of the volcano's crater, but allowed evacuated villagers to walk in, advising them to leave again by nightfall.

"My feeling is it will not blow at this time," a 30-year-old farmer, Budi, said as he returned to cut grass to feed his cows.

Scientists, however, feared an eruption could be imminent for Merapi, which is about 250 miles east of Indonesia's capital, Jakarta.

The mountain, which is one of 129 active volcanoes in Indonesia, sent out a searing cloud of gas that burned 60 people to death when it last erupted in 1994. About 1,300 people died in a 1930 eruption.

The deadly clouds, which contain a mix of hot ash, rock fragments and volcanic gas, are a big worry, said Sugiono, one of the scientists on a team monitoring the volcano 24 hours a day.

He said a glowing dome of lava being formed by magma forced to the surface was poised to collapse and could send searing clouds down the mountain at several hundred miles an hour.

"Hot clouds keep appearing all the time," Sugiono said. "If you get stuck in them, then you have no chance."