The man rolled himself onto the street in a wheelchair and was lying on the ground on his stomach as about 20 officers wearing shields approached him, WSLS-TV, Roanoke, Va., reported Wednesday night.
Police have identified the suspect as Warren "Gator" Taylor of Sullivan County, Tenn.
State Police Sgt. Mike Conroy said three hostages had emerged from the building and officers were checking to make sure nobody else was inside.
Police exercised caution because "we don't know what he had," Conroy said, The Wytheville Enterprise reported.
The man, who authorities initially said was wheelchair-bound, had apparently pushed a wheelchair into the post office in the rural western Virginia town about 2:30 p.m., Conroy said earlier.
Keith Dunagan, chief deputy of the Wythe County Sheriff's Office, said the man had claimed he had two bombs. The man, who has not been identified, had spoken with the FBI, which handled negotiations, WSLS reported.
The man, who said he is a Marine Corps veteran and is not from Wytheville, had allowed a few hostages to call relatives, Dunagan said.
Dunagan said the man had not made demands other than food and to speak with an FBI bomb expert, which he has done. Food was also delivered to the post office, authorities said.
The Enterprise reported Wednesday afternoon that multiple gunshots were heard in the vicinity after the man entered the post office. Police said he had 5 pounds of the plastic explosive C-4 strapped to the wheelchair.
A customer who left the post office as the gunman entered said the man had been driving a red truck with Marine Corps stickers. She said he fired at police and had hand grenades. A sheriff's office employee confirmed there were grenades on the man's truck, the Enterprise reported.