A steady stream of mourners are visiting the scene of the bloody assassination of former Japanese prime minister Shinzo Abe in the western city of Nara, following an unusual act of violence that has shocked the nation.

Japan's longest-serving modern leader was gunned down while making a campaign speech on Friday morning, apparently by a 41-year-old man, in an attack decried by the political establishment as an attack on democracy itself.

"I'm just shocked that this kind of thing happened in Nara," said Natsumi Niwa, a 50-year-old housewife, after offering flowers with her 10-year-old son near the scene of the killing at a downtown railway station.

Abe, a conservative and architect of the 'Abenomics' policies aimed at boosting the Japanese economy, inspired the name of her son, Masakuni, with his rallying cry of Japan as a "beautiful nation", Niwa said. Kuni means nation in Japanese.

A night vigil will be held on Monday, with Abe's funeral to take place on Tuesday, attended by close friends, Japanese media said.

There was no immediate word on any public memorial service.

"I am simply speechless over the news of Abe's death," Prime Minister Fumio Kishida, a protege of the former leader, told reporters on Friday.

"This attack is an act of brutality that happened during the elections - the very foundation of our democracy - and is absolutely unforgivable."

Meanwhile, campaigning resumed on the final day of electioneering before polling for the upper house of parliament, which is expected to deliver victory to the ruling coalition led by Prime Minister Fumio Kishida, a protege of the former leader.

Police, meanwhile, are scrambling to establish the motive behind the attack.

Abe's death has raised questions about the security measures for public figures in Japan, where politicians commonly make direct appeals to voters outside train stations and supermarkets.

A strong election performance "could catalyse Kishida to push for Abe's unfulfilled goal of amending Japan's constitution to allow for a stronger role for the military", James Brady, vice president at advisory firm Teneo, wrote in a note.

Abe became Japan's youngest post-war premier when he took office in September 2006 at the age of 52.

He was rushed to a local hospital in Nara, some 480km west of the capital Tokyo, following the shooting.

He did not regain consciousness and was pronounced dead about five-and-a-half hours after the late-morning attack.

A motorcade believed to be carrying the body of the slain politician left the hospital early on Saturday for his Tokyo residence.

Kishida spoke on Saturday with US President Joe Biden, who expressed his condolences and praised Abe's leadership, broadcaster NHK reported.

Abe was key in the creation of the Quad grouping aimed at countering China's influence in the Indo-Pacific region.

The other members, the United States, India and Australia, expressed shock at the assassination in a joint statement.

"We will honour Prime Minister Abe's memory by redoubling our work towards a peaceful and prosperous region," the statement said.

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