(CNN) Chinese security forces broke into the home of an academic while he was giving a live phone interview with a US broadcaster Wednesday night, abruptly forcing him off-air.

Speaking with Voice of America from his home in the eastern province of Shandong, retired professor Wenguang Sun was discussing Chinese President Xi Jinping's recent visit to Africa when security forces cut him off.

"It's illegal for you to come into my place. I have my freedom of speech," Sun shouted, before the line went dead.

The Chinese government has been stepping up its crackdown on free speech and criticism of the country's leadership in recent years, under an increasingly powerful and assertive Xi.

Former professor Sun Wenguang talking in his home in Jinan, east China's Shandong province on August 28, 2013.

As the police entered his house Thursday, Sun could be heard asking whether he had said something wrong.

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