A loud explosion was heard, but the premier took cover and was unharmed while police subdued a man at the scene, public broadcaster NHK said. There were no injuries, NHK said, citing police, according to Reuters.
"Police are investigating the details of the loud explosive sound at the previous speech venue," Kishida said when he resumed his campaign speeches, in video broadcast by NHK. "I am sorry for causing many people to be concerned. We are in the middle of an important election for our country. We must carry this on together."
Former Prime Minister Shinzo Abe, the longest-serving leader of modern Japan, was assassinated with a homemade gun last July while campaigning for a parliamentary election, shocking the nation and prompting a review of security for politicians, who routinely press the flesh with the public.
By-elections in various regions for the lower house of Japan's parliament are to be held on April 23.
The incident late on Saturday morning incident occurred at the Saikazaki fishing harbor in Wakayama prefecture, some 65 km (40 miles) southwest of Osaka city.
Kishida was being served local specialty seafood just before the explosion, media reported. News video showed Kishida looking behind him in surprise as shouts filled the area.
A man identified by the Asahi newspaper as a staffer of the fishery cooperative grabbed a young man in a headlock as police swarmed the suspect and dragged him to the ground. Moments later, an explosion and cloud of smoke could be seen near where Kishida had been standing.
NHK video showed the thrown object, which appeared to be a metal cylinder.
NHK footage showed crowds running away as several police officers appeared to pin a man to the ground before removing him from the scene. The man appeared to be in his 20s or 30s, media said.
A representative of Wakayama's prefectural police headquarters told Reuters he could not answer questions about the incident. Police arrested a 24-year-old male suspect from Kawanishi city, Kyodo reported.
A woman on the scene told NHK that she saw an object flying overhead and "it gave me a bad feeling, so we ran away unbelievably fast. Then we heard a really loud noise. It made my daughter cry."
Kishida is to host a Group of Seven (G7) summit in Hiroshima next month. Japan's foreign ministry said after the incident there would be no change to the security plan for a G7 foreign ministers' meeting starting on Sunday in the resort city of Karuizawa.
MNA/PR