A man opened fire on Abe (67) from behind with an apparently homemade gun as he spoke at a traffic island in the western city of Nara on Friday.
It was the first assassination of a sitting or former Japanese premier since the days of pre-war militarism in the 1930s.
Speaking before Abe's death was announced, Prime Minister Fumio Kishida condemned the shooting in the "strongest terms" while Japanese people and world leaders expressed shock at the violence in a country in which political violence is rare and guns are tightly controlled.
"This attack is an act of brutality that happened during the elections - the very foundation of our democracy - and is absolutely unforgivable," said Kishida, struggling to keep his emotions in check.
A fire department official had said Abe appeared to be in a state of cardiac arrest when flown to hospital.
Police said a 41-year-old man suspected of carrying out the shooting had been arrested. NHK quoted the suspect, identified as Tetsuya Yamagami, as telling police he was dissatisfied with Abe and wanted to kill him.
Abe was making a campaign speech outside a train station when two shots rang out about 11.30am (local time). Security officials were then seen tackling a man in a grey T-shirt and beige trousers.
"There was a loud bang and then smoke," businessman Makoto Ichikawa, who was at the scene, told Reuters, adding that the gun was the size of a television camera.
"The first shot, no one knew what was going on, but after the second shot, what looked like special police tackled him."
Transfusions
Earlier, Kyodo news service published a photograph of Abe lying face-up on the street by a guardrail, blood on his white shirt. People were crowded around him, one administering heart massage.
Nara emergency services said he had been wounded on the right side of his neck and left clavicle. His brother, Defence Minister Nobuo Kishi, had said Abe was getting blood transfusions.
NHK showed live footage of Abe's wife, Akie, on her way by train to the hospital where he was being treated.
Airo Hino, political science professor at Waseda University, said such a shooting was unprecedented in Japan. "There has never been anything like this."
Senior Japanese politicians are accompanied by armed security agents but often get close to the public, especially during political campaigns when they make roadside speeches and shake hands with passersby.
In 2007, the mayor of Nagasaki was shot and killed by a yakuza gangster. The head of the Japan Socialist Party was assassinated during a speech in 1960 by a right-wing youth with a samurai short sword. A few other prominent postwar politicians were attacked but not injured.
Police said the suspected shooter was a resident of Nara. Media said he had served in Japan's military for three years until 2005. Defence Minister Kishi declined to comment on that.
'Very sad moment'
Earlier US Secretary of State Antony Blinken expressed deep concern over Abe's condition. "Our thoughts, our prayers are with him, with his family, with the people of Japan," Blinken said on the sidelines of a G20 meeting on the Indonesian island of Bali. "This is a very, very sad moment."
The United States is Japan's most important ally.
In a tweet, Australian Prime Minister Anthony Albanese said: "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."
Taiwan's President Tsai Ing-Wen said in a Facebook post: "I believe everyone is as surprised and sad as I am. Taiwan and Japan are both democratic countries with rule of law. On behalf of my government, I would like to severely condemn violent and illegal acts. Former Prime Minister Abe is not only a good friend of mine, but also a staunch friend of Taiwan's. He has supported Taiwan for many years and spared no effort to promote the progress of Taiwan-Japan relations."
Wealthy political family
Abe served two terms as Prime Minister to become Japan's longest-serving premier before stepping down in 2020 citing ill-health. But he has remained a dominant presence over the ruling Liberal Democratic party (LDP) party, controlling one of its major factions.
Kishida, Abe's protege, had been hoping to use the election to emerge from Abe's shadow and define his premiership, analysts have said. Kishida suspended his election campaign after the shooting.
Abe has been best known for his signature “Abenomics” policy featured bold monetary easing and fiscal spending. He also bolstered defence spending after years of declines and expanded the military’s ability to project power abroad.
In a historic shift in 2014, his government reinterpreted the post-war, pacifist constitution to allow troops to fight overseas for the first time since World War 2.
The following year, legislation ended a ban on exercising the right of collective self-defence, or defending a friendly country under attack.
Abe, however, did not achieve his long-held goal of revising the US-drafted constitution by writing the Self-Defence Forces, as Japan’s military in known, into the pacifist Article 9.
He was instrumental in winning the 2020 Olympics for Tokyo, cherishing a wish to preside over the Summer Games, which were postponed by a year to 2021 because of the Covid-19 pandemic.
Abe first took office in 2006 as Japan’s youngest Prime Minister since World War 2. After a year plagued by political scandals, voter outrage at lost pension records, and an election drubbing for his ruling party, Abe quit citing ill-health.
He became Prime Minister again in 2012.
Abe hails from a wealthy political family that included a foreign minister father and a great-uncle who served as premier.
First elected to parliament in 1993 after his father's death, Abe rose to national fame by adopting a tough stance toward unpredictable neighbour North Korea in a feud over Japanese citizens kidnapped by Pyongyang decades ago.
Though Abe also sought to improve ties with China and South Korea, where bitter wartime memories run deep, he riled both neighbours in 2013 by visiting Tokyo's Yasukuni Shrine, seen by Beijing and Seoul as a symbol of Japan's past militarism.
