Thursday marks 30 years since a fast-moving current of hot gas and volcanic matter killed 43 people in Nagasaki Prefecture, southwest Japan.
The pyroclastic flow gushed down Mount Unzen's Fugen Peak on June 3, 1991.
About 150 people took part in a memorial ceremony on Thursday in nearby Shimabara City. They observed a moment of silence for the victims.
Mayor Furukawa Ryuzaburo said the disaster 30 years ago was on an unprecedented scale and left deep scars on people's minds and lives.
He pledged to strengthen disaster-prevention measures, recalling how residents supported each other as they overcame hardships.
The participants left flowers and offered prayers.
Omachi Sumi, who lost her firefighter husband 30 years ago, spoke on behalf of the bereaved families.
She said she is still unable to forget that day when her husband was suddenly taken away forever, and pledged to teach future generations about the horror of volcanic disasters.