He was later brought back, but he refused to answer prosecutors' charges against him, shouting "The Communists use the law of the jungle!" before officers against removed him from the court room.
His display of defiance was striking in a country where dissent is harshly punished.
Ly, 60, who has spent 14 years in prison for his pro-democracy activities - or acting against the state - was accused of producing anti-government documents and communicating with anti-communist groups overseas.
Prosecutors said he resumed his political activities after he was freed from jail in a 2005 amnesty and placed under house arrest.
Passing sentence judge Hiep said Ly deserved "severe punishment" for masterminding efforts to boycott Vietnam's upcoming legislative elections, establish unsanctioned political parties, and overthrow the government.
Hiep said Ly and his co-defendants had committed "very serious crimes that harmed national security".
During the trial prosecutors had said Ly had told police that he "worked day and night" to produce anti-government materials.
They said Ly was a founding member of the outlawed "Bloc 8406" pro-democracy coalition, named after its April 8 launch last year, and was active in the banned Vietnam Progression party.
They also said he had planned to merge with overseas democracy activists to form a new political umbrella group called Lac Hong.
Activists say the case is part of an intensified crackdown by the authorities on civil liberties.
Ly's trial is the first of several expected in the coming months against prominent pro-democracy advocates, including two Hanoi human rights lawyers detained this month.
More than a dozen diplomats and foreign journalists attended Friday's trial.
In an unusual step, foreign observers were allowed to be in the court for the first and last five minutes of the proceedings. They watched the rest of the proceedings on closed-circuit television.