A loud explosion took place near the area where Japanese Prime Minister Fumio Kishida was delivering a speech on Saturday, the NHK TV channel reported.
Kyodo news agency reported that an unknown man threw an object resembling a smoke bomb or a flare gun before Kishida's speech, which then exploded.
Security detained the man and the Prime Minister was evacuated unharmed after the blast.
This came only nine months following the assassination of former Prime Minister Shinzo Abe while delivering a campaign speech in the western city of Nara.
The latest attack also aligns with a series of Group of Seven ministerial meetings begin this weekend ahead of the May 19-21 summit that Kishida will host in Hiroshima.
One witness told NHK she was among the crowd and saw something come flying from behind and that there was a sudden loud noise and she fled with her children.
Another witness revealed that people were screaming and that he saw someone being apprehended right before the explosion occurred.
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