A video shared online reportedly showed him brandishing an object in his hand and saying: "I will not leave this place alive."
ADVERTISEMENTA former Ukrainian army veteran was arrested after he threatened to explode a grenade at the government headquarters in Kyiv.
He had entered the building early on Wednesday, a few hours before a ministerial meeting.
A video shared online reportedly showed him brandishing an object in his hand and saying: "I will not leave this place alive."
Special forces were scrambled. But he surrendered after "ten minutes" of negotiations with one of his friends whom the police had brought to the scene, the national police chief Igor Klymenko said in a statement.
The grenade he was brandishing was real, Klymenko said, adding the veteran faces up to 15 years in prison.
The man had fought in eastern Ukraine against pro-Russian separatists, according to authorities.
In a Facebook post, the Minister of Interior Denys Monastyrsky said “investigative actions will be carried out ... including psychiatric examination.
"During the investigation, it will become clear what prompted a war veteran … to take a grenade and go to the government building.”