Residents of Shimabara, Nagasaki Prefecture, on Sunday paid tribute to the 43 people claimed by the pyroclastic flow triggered by the eruption of Mount Unzen in 1991.
Some 100 people, including students and city officials, laid flowers at a memorial in the town of Nita, in the city's southeast. The city set up stands for residents and family members of the victims to offer prayers and flowers.
After erupting in November 1990 for the first time in around 200 years, the mountain's Fugen Peak developed a lava dome that later collapsed and sent massive currents of hot gas and rocks flowing down its slopes on June 3, 1991.
The 40 confirmed victims included firefighters, geologists and journalists, but the bodies of three were never found.
"The lava dome remains in unstable condition," Shimabara Mayor Ryuzaburo Furukawa said. "We need the relevant government organs to keep monitoring it as we work together with the residents to ensure effective emergency preparedness."