Shopan Pangestu, who filmed the dramatic event as it happened, posted the video on Instagram which showed a huge plume of thick smoke emerging from the volcano and rising into the blue sky just a short distance away from the campers.
Mr Pangestu told Reuters in an email that he and his friends at first heard 'rumbling' sounds and a 'roar', and moments later saw thick plumes of smoke rising from atop the volcano.
Screenshot of the campers reacting with dismay. Source: Reuters
In the video he could be heard reacting to the eruption and saying "astaghfirullah al Azeem" in Arabic, meaning ''I seek forgiveness from god" as he watched ash and debris being spewed in the air.
He said he was frightened and worried for his friend who had climbed to the peak of the mountain.
"We all left our things, heading away north from the mountain. There was a black cloud and we were right under it," Pangestu said.
"Fortunately the direction of the wind was heading south, and the black cloud was southward, not towards us."
The group of campers, who are friends from local universities, rushed back to the basecamp, packed their belongings and prepared to leave.
Pangestu said his friends came down from the top of the mountain feeling "all safe and grateful" to have survived the volcanic eruption.
A disaster mitigation agency spokesperson said that all 120 people who had been hiking up Merapi were safe.
Indonesian authorities ordered people living near the volcano to leave their homes after its eruption spewed steam and ash to as high as 5,500 metre (18,000 ft).
The airport in Yogyakarta, the nearest big city to the volcano, shut because of the threat from the ash, the state-owned aviation agency AirNav said in a statement.
