The National Disaster Risk Reduction and Management Council said ash spewing from the volcano southeast of Manila reached 1,600 feet above the crater.
"At 8 a.m. (Tuesday) Mayon volcano produced a small phreatic event that last about 73 seconds," the NDRRMC said on its website.
Gray and brown clouds reached 1,600 feet above the summit, the agency said. The hikers apparently were caught in the ash eruption and falling rocks.
"A total of 27 hikers were on the volcano and requested assistance for rescue," the agency said. "As of reporting time, a total of 10 casualties were reported (five dead, five injured) while six individuals were reportedly unharmed."
The dead included four Germans and one Filipino tour guide, the agency said.
Local reports said seven people were injured.
A phreatic explosion generally involves eruptions of steam, water, volcanic rocks and ashes, but without magma or lava, said Renato Solidum Jr., director of the Philippine Institute of Volcanology and Seismology, The Philippine Star reported.
"What happened to Mayon Volcano this morning is essentially a hydrothermal or steam-driven explosion," Solidum said.
His agency urged people not to enter a 3.5-mile radius of the volcano's permanent danger zone because of the threat of steam and rock falls from the volcano, the Star reported.