Fumio Kishida said health workers are “working very hard” to handle the situation.
Abe, 67, was delivering a speech at a political event on a street in the western city of Nara when he was "shot from behind” by a person who has now been arrested.
The incident, termed "deplorable" by the government spokesperson, took place at around 11:30 a.m. local time (0230GMT).
"Emergency officials say he does not appear to have any life signs," according to Japanese public broadcaster NHK.
Japan will hold senate elections on Sunday and Abe was campaigning for the ruling Liberal Democratic Party (LDP) candidates.
At least two shots were heard before Abe collapsed and fell to the ground. He was bleeding and the gunman shot him from behind, witnesses told the NHK.
He was “unconscious and unresponsive.”
The assailant has been identified as 41-year-old Tetsuya Yamagami, a resident of the city of Nara. The shotgun used in the attack has been seized.
Abe served Japan as premier for the longest time – first in 2006 and then again from 2012 to 2020.
Abe, then 52, was elected head of the LDP on Sept. 20, 2006.
In six days, he rose to become the 90th prime minister of Japan, which has the world’s third-largest economy. He already had experience under his belt serving as the government’s chief cabinet secretary from 2005 to 2006.
He was Japan’s youngest premier in the postwar era, a record that still stands.
However, the first jolt to his political career came when he suddenly resigned due to a bad ulcerative colitis illness in 2007. He had been in office for just a year – September 2006 to September 2007.
Abe made a return to power in 2012, first defeating party rival Shigeru Ishiba in September to retake the LDP helm, and then leading the party to an overwhelming majority that December.
It was again a political milestone – the first Japanese former premier to return to office since Shigeru Yoshida in 1948.
He was re-elected in the 2014 and 2017 elections, and stepped down in 2020 due to health issues.