When Chinese police beat elderly former professor Sun Wenguang so badly they broke four ribs, it was just part of the price he has paid for a lifetime defying authority.

The white-haired, soft-spoken Sun first went to prison nearly 50 years ago and is now one of China's oldest activists, admired by the younger generation for his doggedness.

He turns 80 next month but still persists in simple acts of protest, dodging minders in a risky game of cat and mouse.

"I am, in their view, a bastard who just won't stop," he says, chuckling in his study late one night after his monitors had left.

"If my rights are infringed then I have to fight back. I can't just give up my rights."

Sun is kept under regular surveillance by Chinese authorities, with a round-the-clock guard posted at his apartment at sensitive times.

During the trial of former politician Bo Xilai in his home town of Jinan last month, a minder planted himself directly outside the door to prevent it even opening.

It was the sufferings caused by the Mao Zedong's disastrous 1958-61 Great Leap Forward and 1966-76 Cultural Revolution that turned Sun, then a university teacher of physics and economics, against the Communist party.

Like many he was branded a "counter-revolutionary" and locked in makeshift campus jails.

But he refused to admit wrongdoing and posted signs at the university entrance defending himself and others being punished.

When one was torn down he would post another, sending him to detention twice more for three years total.

"Whenever I was let out I would write," he says. "They used to say about me afterward, whoever is down, Sun defends."

For seven years from 1974 Sun was detained in labour camp and prison after he was convicted of being a counter-revolutionary -- giving him more time to ponder the country's problems.

"How could society be improved, the situation be changed, to avoid producing so much suffering in the future? I couldn't stop thinking about this," he says.

"If the nation and society have such huge problems and you only try to solve your own, it won't work."

When Sun was released in 1981 China had begun to open up, only to crack down on Tiananmen Square democracy protestors in 1989.

Every year Sun tries to outwit his minders to mark the June 4 Tiananmen anniversary, and the death of former leader Zhao Ziyang who was ousted after sympathising with the demonstrators.

It was his attempt to honour Zhao in 2009 that provoked the vicious beating. He sneaked past guards swarming his building and made it to a park before being dragged aside and thrashed.

"Even with lots of people around that didn't stop them," he says.

Undeterred, Sun escaped his guards ahead of the Tiananmen anniversary this year by going out for a swim every day until they stopped bothering to follow him.

That freed him to meet friends on an agreed date and quickly unfurl a banner in public.

"Seeing this of the older generation of activists is a huge encouragement," says Hu Jia, a prominent Beijing-based dissident.

"Professor Sun -- for both his thinking and his actions -- is an icon for Chinese dissidents."

Sun has poured his political views into books criticising Communist party rule and advocating free elections.

His first in 2002 collected essays he scribbled in jail, some on toilet paper with a reed plucked from his straw bedding and dipped in soy sauce.

After his second book came out in Hong Kong two years later, he was banned from travelling outside the mainland.

He has yet to see his latest work -- about his attempts to run for office -- after a friend had a copy confiscated while bringing it into the mainland.

In 2007 and 2011 Sun tried to win a seat on the local legislature, fully aware that the rubberstamp body was not holding genuine elections.

Police tore down his posters and, the second time, banned him from campaigning at his former campus.

Instead he crept in at odd hours to make speeches to whatever students he could find.

Sun belongs to the older school of Chinese activists who directly provoke the authorities with calls for democracy, whereas the less-confrontational newer generation tries to uphold existing laws, says Maya Wang, an Asia researcher at Human Rights Watch.

"He is respected for his long years of activism and the fact that he has persisted in the face of both harassment and physical threats as well as house arrest," she says.

"From his generation it's very rare to have somebody who continues to do this."

Sun believes that if more Chinese also press for change, the party will eventually have to bow to democracy -- treacherous words in the tightly controlled state.

"Political reform is necessary for the Communist party... If they don't do it right then they may not survive," he says.

"The way forward will ultimately be decided not by the party but by the people."