Article has been updated to include statement from Department of State.

A Venezuelan judge on Thursday sentenced Leopoldo López, arguably the country's most prominent opposition figure, to more than 13 years in prison on charges of inciting anti-government riots that led to the deaths of more than 40 individuals last year.

López, a U.S.-educated former mayor of Caracas' Chacao district, has been one of the most vocal critics of embattled President Nicolás Maduro and his late predecessor, Hugo Chávez. The leader of the Voluntad Popular (Popular Will) party had been jailed since February 2014.

Judge Susana Barreiros, who also established the guilt of student leaders Cristian Holdack, ángel González and Demián Martín, ordered for López to remain at the the Ramo Verde military prison, El Nacional reported.

As the opposition leader's sentences was being determined during a marathon session in the Venezuelan capital, supporters and pro-government clashed outside the heavily guarded Palace of Justice, Reuters explained. One protester, a man identified as Horacio Blanco, was killed during the incidents when he suffered a heart attack, the semi-official Telesur television network added.

Barreiros revealed her sentence at 9:30 p.m. local time, El Universal noted, and the judge now has 10 days to publish the motives that led to her decision. Around Caracas, meanwhile, outraged López allies banged pots and pans -- a traditional form of protest in Venezuela, according to Reuters.

Jesús Torrealba, who leads the opposition coalition that includes López's Popular Will party, called the former mayor a political prisoners as he decried the judge's decision.

"This is not only an attack on liberty and legality ... but also a provocation to the Venezuelan people," he insisted.

Prosecutors accused López of "calling for the removal of the democratically elected president" and inciting violence in the 2014 incidents, but the opposition figure claims that Maduro's government is trying to restrict Venezuelans' freedom of expression and criminalize anti-government speech. His lawyer, Elenis Rodríguez, meanwhile warned Barreiros of her verdict's potential consequences.

"You are more afraid of handing down this sentence than I am to hear it," Rodríguez said in a lengthy final statement, according to El Universal.

The U.S. has expressed concern following the news of López's imprisonment. Assistant Secretary of State for Western Hemisphere Affairs Roberta Jacobson, who oversees Latin America for the State Department, sent out a tweet in both Spanish and English.

Sen. Marco Rubio, R-Fla., who has criticized the Obama administration for their slow response to issues in Venezuela and vehemently opposes the Maduro government, released a statement in solidarity with López.

"Leopoldo Lopez's arrest, incarceration, show trial and prison sentence have all been a sham. The Venezuelan regime is robbing Leopoldo of his freedom, his wife of a husband, his kids of a father and the Venezuelan people of a leader committed to their democratic aspirations. He deserves to be freed immediately and unconditionally," he stated.

Sen. Rubio also criticized the administration's slow response to the problems in Venezuela and called for action on the Venezuela Defense of Human Rights and Civil Society Act of 2014, which he introduced.

The Department of State published a press release today from Secretary of State John Kerry. Kerry expressed the country's dismay at the news of Leopoldo's conviction and questioned its validity, claiming the Venezuelan courts were being used to "suppress and punish government critics."

"Since Mr. Lopez's arrest and imprisonment in February of 2014, we have underscored our concern with the charges brought against Mr. Lopez -- which we consider illegitimate -- and we have repeatedly called for his release and for the release of all Venezuelans who are imprisoned for political reasons," Kerry said.

"We call on the Government of Venezuela to respect the rights of all political prisoners; and to guarantee fair and transparent public trial, consistent with the American Declaration of the Rights and Duties of Man, the Inter-American Democratic Charter, the International Covenant on Civil and Political Rights, and Venezuela's Constitution."