A man (C) believed to be a suspect who threw a pipe-like object near Fumio Kishida (R) during his outdoor speech, is held by police in Wakayama on Saturday. AP / Reuters

Japanese Prime Minister Fumio Kishida resumed campaigning on Saturday after being evacuated unharmed from the scene of an apparent "smoke bomb" blast.

The incident in western Japan's Wakayama came less than a year after the assassination of former prime minister Shinzo Abe, which traumatised the country and forced a security shake-up for public officials.

Kishida was in the city to deliver remarks in support of a ruling party candidate at a local port when a disturbance rippled through the crowd gathered to hear him speak.

Footage from national broadcaster NHK showed the prime minister turning to look backwards as a person was detained by security and people moved away, some shrieking.

Seconds later, a blast was heard and white smoke filled the air. Footage and photos from the scene showed a silver, pipe-like object on the ground, but it was not immediately clear whether it had caused the blast and smoke.

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Government officials said a man had been arrested on suspicion of obstruction of business. He has been identified as a 24-year-old man from the Hyogo region, a Wakayama police official told AFP. There was no immediate information on a potential motive. People at the scene described moments of panic.

"I ran frantically, and then, 10 or so seconds later, there was a loud sound and my kid started crying. I was stunned, my heart is still beating fast," one woman told NHK.

Former Prime Minister Shinzo Abe, the longest-serving leader of modern Japan, was assassinated with a homemade gun last July while campaigning for a parliamentary election, shocking the nation and prompting a review of security for politicians, who routinely press the flesh with the public.

Smoke billows after an explosion was heard near Fumio Kishida's outdoor speech in Saikazaki. Reuters

By-elections in various regions for the lower house of Japan's parliament are to be held on April 23.

The incident late on Saturday morning incident occurred at the Saikazaki fishing harbour in Wakayama prefecture, some 65km southwest of Osaka city.

Kishida was being served local speciality seafood just before the explosion, media reported. News video showed Kishida looking behind him in surprise as shouts filled the area.

A man identified by the Asahi newspaper as a staffer of the fishery cooperative grabbed a young man in a headlock as police swarmed the suspect and dragged him to the ground. Moments later, an explosion and cloud of smoke could be seen near where Kishida had been standing.

NHK video showed the thrown object, which appeared to be a metal cylinder.

A man, on the ground, is caught after what appeared to be a smoke bomb was thrown at a port. AP

A representative of Wakayama's prefectural police headquarters told Reuters he could not answer questions about the incident. Police arrested a 24-year old male suspect from Kawanishi city, Kyodo reported.

A woman on the scene told NHK that she saw an object flying overhead and "it gave me a bad feeling, so we ran away unbelievably fast. Then we heard a really loud noise. It made my daughter cry."

Kishida is to host a Group of Seven (G7) summit in Hiroshima next month. Japan's foreign ministry said after the incident there would be no change to the security plan for a G7 foreign ministers' meeting starting on Sunday in the resort city of Karuizawa.

Agencies