Japan announced the death of former Japanese Prime Minister Shinzo Abe after he was shot at a campaign event on Friday, a government spokesperson said, as local media reported the nation's longest-serving premier was showing no vital signs.

Earlier, Japanese Prime Minister Fumio Kishida revealed Friday that former Prime Minister Shinzo Abe is in a "very critical condition" after he was shot.

"I pray that former Prime Minister Abe may survive," Kishida said. "It is a barbaric act during the electoral campaign that is the basis of democracy. It is absolutely unforgivable and I condemn it in the strongest terms," he added.

"Former prime minister Abe was shot at around 11:30 am," in the country's western region of Nara, chief cabinet secretary Hirokazu Matsuno told reporters.

"One man, believed to be the shooter, has been taken into custody. The condition of former prime minister Abe is currently unknown."

"Whatever the reason, such a barbaric act can never be tolerated, and we strongly condemn it," Matsuno added.

Local media including national broadcaster NHK and the Kyodo news agency said Abe appeared to be in "cardiorespiratory arrest", a term often used in Japan before a feared death can be officially confirmed by a coroner.

The attack on a man who may be Japan's best-known politician comes despite the country's famously low levels of violent crime and tough gun laws, and with politicians campaigning ahead of upper house elections on Sunday.

Abe, 67, had been delivering a stump speech with security present, but spectators were able to approach him fairly easily.

Footage broadcast by NHK showed him standing on a stage when a loud blast was heard with smoke visible in the air.

As spectators and reporters ducked, a man was shown being tackled to the ground by security.

Local media identified the man as 41-year-old Tetsuya Yamagami, citing police sources, with several media outlets describing him as a former member of the Maritime Self-Defense Force, the country's navy.

- 'A large bang' -

Witnesses at the scene described shock as the political event turned into chaos.

"He was giving a speech and a man came from behind," a young woman at the scene told NHK.

"The first shot sounded like a toy bazooka. He didn't fall and there was a large bang. The second shot was more visible, you could see the spark and smoke," she added.

"After the second shot, people surrounded him and gave him cardiac massage."

Abe was bleeding from the neck, witnesses said and photographs showed. He was reportedly initially conscious but subsequently lost consciousness, NHK reported.

An official at Nara Medical University hospital told AFP: "What we can share now is that his transfer here has been completed," declining to comment on Abe's status.

Officials from the local chapter of Abe's Liberal Democratic Party said there had been no threats before the incident and that his speech had been announced publicly.

Prime Minister Fumio Kishida, himself on the campaign trail, was returning to Tokyo and expected to speak upon arrival.