A smoke bomb exploded as Japanese Prime Minister Fumio Kishida was due to begin an election campaign speech on Saturday.
Kishida was not hurt in the incident in Wakayama, western Japan, the Japanese television station NHK and news agency Kyodo reported.
Kyodo said a man was apprehended by police after the incident and Kishida's speech, to support his ruling party's candidate in a local election, was cancelled.
In July, former Japanese prime minister Shinzo Abe was fatally shot during a campaign speech in the city of Nara.
Kishida was unharmed, and soon resumed campaigning, including a stop at a local train station.
"There was a loud blast sound at the previous speech venue. Police are investigating details, but I'd like to apologise for worrying many people and causing them trouble," he said.
"An election that's important to our country is taking place, and we must work together and follow through on it."
He was also due to appear at a stop later Saturday afternoon in Chiba, east of Tokyo.
"That something like this happened in the middle of an election campaign that constitutes the foundation of democracy is regrettable. It's an unforgivable atrocity," Hiroshi Moriyama, the ruling Liberal Democratic Party's election strategy chairman, told NHK.
Security at local campaign events in Japan can be relatively relaxed, in a country with little violent crime and strict gun laws.
