The head of Japan's National Public Safety Commission says VIP protection at election campaign speech events is being carefully planned in advance since last year's fatal shooting of former Prime Minister Abe Shinzo.

On Saturday, an explosive device was thrown at Prime Minister Kishida Fumio as he was about to give a speech in support of a by-election candidate in the western Japanese city of Wakayama. Kishida escaped unharmed.

The attack occurred after VIP protection arrangements were drastically changed in the wake of Abe's shooting.

Commission Chair Tani Koichi said such acts of violence can never be allowed, as elections are a foundation of democracy.

Tani said the case would be thoroughly investigated and security arrangements constantly reviewed.

He has instructed police nationwide to boost security, including the deployment of more officers, questioning suspicious individuals and conducting baggage checks.

Tani, himself a ruling party lawmaker, admitted the difficulty of protecting VIPs at campaign speech events.

He said police negotiate security arrangements with parties and their candidates, while understanding that they are eager to mingle with as many voters as possible.

Tani said he will promote clear communication between the police and political parties to ensure security at such events.