BEIJING — As he was carried away by Chinese authorities, Wenguang Sun could only say: “I am entitled to freedom of speech.”

The retired Chinese professor and activist was being interviewed by U.S.-funded Voice of America when police officers burst through the door of his apartment. Sun was criticizing Chinese leader Xi Jingping’s billion-dollar investment in African infrastructure, arguing that Xi had ignored the poor in his own country.

Viewers listened as Sun recounted his ordeal live on air. “Here they come again, the police are here to interrupt again,” Sun said in Chinese. “Four, five, six of them.”

Sun asked what the men were doing in his home and threatened to get a knife. “It is illegal for you to come to my home,” he said. He defended his VOA interview and called on the security officials to respect his rights.

Then the line went dead.

Sun has not been heard from since Wednesday. Inquiries into his whereabouts from VOA have gone unanswered, the outlet said. Calls to Sun’s wife were not returned.

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It’s not the first time the octogenarian has been in trouble with the law. Sun graduated from Shandong University in 1957. He went on to teach economics and physics at the school, about 300 miles south of Beijing, for nearly four decades.

Like many intellectuals, Sun was detained in a makeshift campus prison during the 1960s. In 1974, he was sentenced to seven years in prison for “attacking Great Leader Chairman Mao.” Sun’s crime? Posting signs at the university defending himself and others.

In the years since, he has kept up his criticism of China’s Communist Party. In 2004, Sun published “A Century of Disasters: From Mao Zedong to Jiang Zemin” in Hong Kong. Four years later, he was one of 303 intellectuals to sign on to Charter 08, a public petition calling for the democratization of China.

This July, he took an even bolder step, targeting Xi directly in an open letter. The online missive took Xi to task for his “checkbook diplomacy.” In particular, Sun attacked China’s signature Belt and Road investment project, which is funneling millions of dollars to African infrastructure projects. He also criticized Xi for ditching term limits.

Sun’s attacks have made him a target of the government. In 2005, his passport application was denied, and he has been unable to leave the country since. Four years later, he was beaten while visiting the grave of a dissident, ending up with four broken ribs. In March, his pension was cut. According to Voice of America, Sun was already under house arrest when the police officers burst through his door. In 2013, he told the South China Morning Post that guards are posted at his door “round-the-clock” during “sensitive times.”

Despite the government pressure, Sun has vowed to keep fighting. “If my rights are infringed then I have to fight back,” Sun told the South China Morning Post. “I can’t just give up my rights.”

Patrick Poon of Amnesty International in Hong Kong called Sun’s arrest “shocking and outrageous.”

“It’s another example of how the Chinese authorities are determined to silence dissidents,” Poon said in an email. “I’m very worried about his whereabouts.” Poon also noted that Sun’s arrest during an interview was rare. “Usually, they would just warn the activist after they have talked to the media.”

Hu Jia, a prominent human rights activist in Beijing who appeared on VOA with Sun, echoed Poon’s surprise.

“They did that on air, and they didn’t care if it was in front of the whole world,” Hu said. “This is an attack on press freedom, too. It just shows that they are willing to pay any price" to silence him.

“No matter what Xi Jinping says about rule of the law, this is a clear evidence [against his claim],” he added. “Do those people have no fathers?”

Luna Lin and Emily Rauhala contributed to this report.