Japanese Prime Minister Fumio Kishida was evacuated after a loud explosion was heard just before he began his speech in Wakayama.
According to Japanese media, a smoke or pipe bomb was thrown at the venue where he was just about to begin a speech.
The video footage showed people who had gathered at the scene running for shelter, while a man was subdued by several others who appeared to be police officers at the scene.
The incident comes as Group of Seven climate and energy ministers meet in the northern city of Sapporo, and a day before G7 foreign ministers arrive in Karuizawa in Nagano for talks.
Japan will host the G7 leaders' summit next month in Hiroshima.
The incident comes less than a year after former Prime Minister Shinzo Abe was shot dead while on the campaign trail in July 2022, which traumatised the country and forced a security shake-up for public officials.
His alleged assassin, Tetsuya Yamagami, reportedly targeted him over his links to the Unification Church, and the incident sparked revelations about the connection between the sect and political figures in Japan.
