HANOI, Vietnam — A court in Vietnam on Monday sentenced a United States citizen to 12 years in prison after finding him guilty of attempting to overthrow the state, in a trial that lasted just half a day, his lawyer said.

Michael Phuong Minh Nguyen, 55, pleaded guilty, his lawyer, Nguyen Van Mieng, said by telephone after the trial in Ho Chi Minh City. “It’s such a long sentence,” Mr. Mieng said. “Michael admitted guilt at the trial and asked the jury to reduce his sentence so that he could soon reunite with his family.”

Mr. Nguyen was accused of inciting Vietnamese people to join protests and of trying to attack government offices in Hanoi and Ho Chi Minh City with Molotov cocktails and slingshots, the state-run Tuoi Tre newspaper reported, citing the indictment.

“We are disappointed by today’s verdict,” a spokeswoman for the United States Embassy in Hanoi said. “We will continue to raise our concerns regarding Mr. Nguyen’s case, and his welfare, at all appropriate levels.”