Japan's Prime Minister Kishida Fumio has called on police to do everything they can to ensure security for election campaigning and the Group of Seven summit in Hiroshima next month.

Kishida made the appeal on Sunday, a day after a suspected explosive device was thrown in and blew up at the site where he was just about to give a speech.

Kishida was there to lend support to a Lower House by-election candidate in the western city of Wakayama. The prime minister was evacuated unharmed. One police officer was slightly wounded.

Kishida said he can never tolerate violence during an election campaign, because it threatens the very foundation of democracy.

He said it is important to allow elections to happen as scheduled and that he wants police to do all they can to prevent obstruction of campaigning by every party.

Voters across Japan will go to the polls on April 23, when the second round of unified local elections and five by-elections for the Diet will take place.

A reporter asked Kishida whether he thinks security arrangements for his speech venue were sufficient in light of the coming G7 summit.

Kishida said the security measures will be examined while an investigation into the incident proceeds.

He added that maximum security steps must be in place for the G7 summit and other gatherings that bring together dignitaries from around the world.