"Former prime minister Abe was shot at around 11:30 am in Nara. One man, believed to be the shooter, has been taken into custody. The condition of former prime minister Abe is currently unknown," chief cabinet secretary Hirokazu Matsuno told reporters.
Japanese media said Abe was in a state of cardiopulmonary arrest following the shooting. National broadcaster NHK quoted firefighters as saying he had no vital signs.
The broadcaster said a man in his 40s had been arrested for attempted murder and a gun had been confiscated from him, citing police sources.
The former leader had been delivering a stump speech at an event ahead of Sunday's upper house elections when the apparent sound of gunshots was heard, NHK and the Kyodo news agency said.
"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. 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, 67, collapsed and was bleeding from the neck, a source from his ruling Liberal Democratic Party told the Jiji news agency.
NHK and Kyodo both reported Abe was taken to hospital and appeared to be in cardo-respiratory arrest -- a term used in Japan indicating no vital signs, and generally preceding a formal certification of death by a coroner.
Several media outlets reported that he appeared to have been shot from behind, possibly with a shotgun.
The government said a task force had been formed in the wake of the incident and the top government spokesman was expected to speak shortly.
Abe, Japan's longest-serving prime minister, held office in 2006 for one year and again from 2012 to 2020, when he was forced to step down due to the debilitating bowel condition ulcerative colitis.
"Shocking news from Japan that former PM Shinzo Abe has been shot -- our thoughts are with his family and the people of Japan at this time," Australia's Prime Minister Anthony Albanese wrote in a tweet.
Shocking news from Japan that former PM Shinzo Abe has been shot - our thoughts are with his family and the people of Japan at this time
— Anthony Albanese (@AlboMP) July 8, 2022
Japan has some of the world's toughest gun-control laws, and annual deaths from firearms in the country of 125 million people are regularly in single figures.
Getting a gun licence is a long and complicated process even for Japanese citizens, who must first get a recommendation from a shooting association and then undergo strict police checks.
(FRANCE 24 with AFP)