Image copyright Getty Images Image caption Prof Sun is a well-known critic of the Chinese government

"I have my freedom of speech," are the last words a retired university professor is heard saying before the line goes dead.

On Wednesday, Sun Wenguang, 84, was in the middle of an interview with US-funded broadcaster Voice of America (VOA) when police broke into his home in Jinan, China and forced him off air.

Prof Sun has in the past been openly critical of the Chinese government.

A friend confirmed to the BBC that Prof Sun had been taken from his home by Jinan city police officers.

VOA says it has not been able to reach Prof Sun since then.

What was he saying in the interview?

Prof Sun had been talking to the Mandarin language service of VOA about Chinese government's foreign investments

It followed an open letter he wrote recently criticising President Mr Xi's decision to spend money on foreign aid, loans and investments. He urged him instead to concentrate on domestic spending.

The letter also criticised Mr Xi's decision to remove presidential term limits.

As he is talking, voices can be heard in the background of the recording, which VOA has since shared on Twitter.

"Here they are again - seven, eight [of them]," he tells the interviewer before addressing the group of officers. "What, did I say anything wrong? Listen to what I say, is it wrong?"

He goes on again to explain his criticism of China's foreign investments: "People [in China] are poor. Let's not throw our money in Africa. Throwing money like this is of no good to our country."

Prof Sun's shouts then get louder as he can be heard telling the policemen: "What are you doing? It's illegal for you to come to my home. I have my freedom of speech."

The line then abruptly goes dead.

Who is Sun Wenguang?

A retired physics professor from Shandong University, he spent more than a decade in and out of prison at various times from the 1960s to the 1980s for criticising communist leader Mao Zedong.

The long-time government critic is one of the original signatories of "Charter 08", a manifesto which called for political change in China.

In 2009, Prof Sun was beaten while visiting the grave of Zhao Ziyang, a communist leader who was purged for supporting the Tiananmen protests of 1989.

The then 75-year-old said at the time he had suffered three broken ribs and injuries to his hands and legs. He was later admitted into hospital.

Prof Sun has also been denied a passport, according to the New York Times, and so is unable to leave the country.

What has happened since the interview?

It's unclear. VOA says its attempts to reach Prof Sun have been unsuccessful and there has been no official confirmation of any arrest or police action.

But a friend of Prof Sun, who confirmed he was taken away by police officers, told the BBC that she believes he and his wife are being held in a local hotel, where he has been detained previously.

Li Hongwei added that she believed Prof Sun was within his rights to express what he thought about the government.

According to Human Rights Watch, Mr Sun's experience is the "daily reality" of a human rights activist in China.

"At any moment, police officers can [arrive] to take them away to be interrogated, detained, tortured or mistreated, simply for challenging the authorities' narratives and speaking to foreign media," senior researcher Maya Wang told the BBC.

Patrick Poon, an East Asia researcher from Amnesty International echoed these remarks: "It's a disgrace to see how a Chinese public intellectual who was [doing] a media interview was suddenly cut off by police officers."

"It vividly shows how the Chinese authorities clamp down on free speech," he told the BBC. "Police can harass [dissidents] any time and anywhere they like."