Video footage released by Japan’s public broadcaster, NHK, showed members of the public fleeing and a man being arrested following the incident.
Kishida was evacuated from the venue in the city of Wakayama where he was expected to give an outdoor speech, NHK reported.
NHK said an “explosion-like sound was heard on the scene”.
According to an NHK reporter on the scene, it was “believed that something was thrown in and exploded” just before Kishida was about to begin his speech.
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Japanese officials say the prime minister is safe and unharmed, according to NHK.
A man believed to be a suspect was apprehended at the scene, and NHK footage showed several uniformed and plainclothes police officers gathered around the man and pressing him to the ground.
The video also shows members of the public running away in panic during the evacuation.