Japan's Prime Minister Fumio Kishida has vowed to boost security as ministers from overseas visit the country, after an apparent smoke bomb was thrown at him.

He was evacuated unharmed on Saturday from a public event, with witnesses describing a person throwing an object, followed by smoke and a loud bang.

Japan hosts ministers from the world's seven richest nations on Sunday.

Mr Kishida said the country must "maximise its efforts" on security.

"At a time when high-ranking officials from all over the world are visiting... Japan as a whole needs to maximise its efforts to ensure security and safety," Mr Kishida told reporters on Sunday.

"It's unforgivable such a violent act was committed during an election campaign," he added.

Also on Sunday, police said a 24-year-old suspect detained at the scene had been carrying a knife and a possible second explosive device.

After the incident, which happened during a campaign event in Wakayama, Mr Kishida addressed a crowd in another location, saying: "I'd like to apologise for worrying many people and causing them trouble."

Video from Japan's public broadcaster NHK showed officers piling on top of a person, believed to be the suspect, as people ran away from the scene.

A dramatic photo showed the moment the device flew through the air.

The person was arrested on suspicion of obstruction of business and later identified by authorities as 24-year-old Ryuji Kimura. The motivation for the apparent attack is still unclear.

He was carrying a knife when he was arrested and a possible second explosive device that he dropped after bystanders and police tackled him, Kyodo news agency reported.

Violent attacks are extremely rare in Japan. But there is nervousness about security around politicians after former Prime Minister Shinzo Abe was shot dead while on the campaign trail last year. (BBC)

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