A vocal critic of China's government has disappeared without a trace after security forces strong-armed their way into his home in the middle of a phone interview with a US-funded television network. Retired academic Sun Wenguang, who is in his mid-80s, was speaking live to a Chinese-language TV show for Voice of America (VOA) when the authorities suddenly showed up. "The police are here to interrupt again," Sun said from his home in Jinan in the eastern province of Shandong, counting as many as eight intruders as he spoke in an audio broadcast on Wednesday. "It's illegal for you to come to my home. I have my freedom of speech!" are the final words heard from Sun. The dramatic recording highlights the increasing intensity of China's efforts to silence critics of its policies as the ruling Communist Party aggressively nurtures a cult of personality around President Xi Jinping. Sun had written an open letter to Xi last month that criticised China's chequebook diplomacy in Africa, releasing it just as the leader embarked on a trip to the continent, according to VOA and online screenshots that could not be independently verified by AFP. "Listen to what I say, is it wrong?" Sun asked the security detail in the recording. "People are poor. Let's not throw our money in Africa," he said, telling the intruders that "throwing money like this is of no good to our country and society" before the line went dead. - 'Real tyranny' - Sun, who is one of China's oldest activists, is kept under regular surveillance. He was a co-signer of the pro-democracy manifesto Charter 08 -- a quickly censored document that landed co-author and Nobel Peace Prize winner Liu Xiaobo in prison. Liu died last year, the first Nobel winner to die in custody since Nazi Germany. In 2009, Sun was viciously beaten by authorities when he snuck past guards watching his building in an attempt to pay his respects to ousted Communist leader Zhao Ziyang -- who opposed the use of force to quell the 1989 Tiananmen Square democracy protests -- on the fourth anniversary of the former premier's death. Repeated calls by AFP to his mobile and home phone number went unanswered Friday, as did messages sent via social messaging app WeChat. Calls to the Jinan public security bureau and the publicity department of Shandong University, his former employer, also yielded no response. "Every time you hear overblown rhetoric about how we are on the verge of 'tyranny' or 'authoritarianism' in America remember what real tyranny and authoritarianism looks like," said US Senator Marco Rubio of the incident on Twitter. VOA is "monitoring the situation closely and will provide an update to program viewers once more information becomes available," its spokeswoman Bridget Serchak said in a statement.