“At approximately 2.15pm today, a person has taken hostages and holed up at a post office in Chuo 5-chome area of Warabi city…The perpetrator is possessing what appears to be a gun,” the city’s authorities said on their website.
“Citizens near the scene are urged to follow police instructions and evacuate in accordance with police instructions.”
The Yomiuri daily reported that around 10 post office staff members may be inside the building.
The Yomiuri also reported there is speculation that the man may be carrying kerosene with him, without citing sources.
Police urged 300 residents in the nearby area to evacuate, broadcaster TBS said.
A number of police cars were surrounding the post office.
The incident came as police investigated a shooting incident in the nearby town of Toda, on the outskirts of Tokyo, earlier today.
Two people were slightly wounded in that incident, but it was unclear how they were hurt.
It was also unclear if the two incidents were related.
Images on television showed the man inside the post office in a baseball cap and a white shirt under a dark coat, with what looked like a gun attached to a cord around his neck.
Violent crime is astonishingly rare in Japan, in part because of strict regulations on gun ownership.
Japan has one of the lowest murder rates in the world.
However, recent years have seen violent crimes, including gun attacks, make headlines in the country, most notably the assassination of former prime minister Shinzo Abe in July last year.
Abe’s accused assassin, Tetsuya Yamagami, reportedly targeted the politician over his links to the Unification Church.
In April, a man was arrested for allegedly hurling an explosive towards Prime Minister Fumio Kishida as he campaigned in the city of Wakayama. Kishida was unharmed.
The following month, a man holed up in a building after allegedly killing four people, including two police officers and an elderly woman, in a gun and knife attack.
Masanori Aoki, 31, was taken into custody at his house outside a farm near the city of Nakano in the Nagano region, police said at the time.
Two women and two police officers were killed in the attack.