There were no immediate reports of casualties, said Ahmad Rifandi, an official with Indonesia's Volcanology and Geological Hazard Mitigation Center at the Marapi monitoring post.
The two routes for climbers were closed after the eruption and villagers living on the slopes of the mountain were advised to stay three kilometres from the crater's mouth because of potential lava.
About 70 climbers started their way up the nearly 2900m mountain on Saturday and became stranded. So far, 49 have been successfully evacuated with the rest still awaiting rescue, said Hari Agustian, an official at the local Search and Rescue Agency in Padang, the capital city of West Sumatra province.
He said about 168 rescuers, including police and soldiers, have been deployed to rescue all the climbers.
A video on social media shows the climbers were evacuated to a shelter, their faces and hair smeared with volcanic dust and rain.
National Disaster Management Agency spokesperson Abdul Muhari said several villages were blanketed with falling ash, blocking out the sun in many areas. Authorities distributed masks and urged residents to wear eyeglasses to protect them from volcanic ash, he said.
About 1400 people live on Marapi's slopes in Rubai and Gobah Cumantiang, the nearest villages about 5-6km from the peak.
Marapi's alert level was maintained at the third-highest of four levels, Abdul Muhari said, and confirmed that authorities had been closely monitoring the volcano after sensors picked up increasing activity in recent weeks.
Marapi has been active since January when it also erupted without causing casualties. It is among more than 120 active volcanoes in Indonesia, which is prone to seismic upheaval due to its location on the Pacific Ring of Fire, an arc of volcanoes and fault lines encircling the Pacific Basin.
