Dramatic video footage emerged Monday of one of China's top dissidents breaking through a security cordon to reach the wife of jailed Nobel laureate Liu Xiaobo, who is herself under house arrest.

The four-minute 12-second video, posted on YouTube, is shot in Liu Xia's apartment building in Beijing and represents a daring affront to the authorities.

It also illustrates the level of fear under which she lives, according to activists who say her detention is illegal.

A security official is shown telling leading dissident Hu Jia, who first came to prominence fighting for the rights of victims of an HIV scandal, and two other activists: "Who are you looking for?"

Told they want to see Liu Xia, who has been held under house arrest since her husband won the peace prize in October 2010, he replies firmly: "No, no, that's not possible."

"Who are you to tell us it's not possible?" comes the response, before the group push past him, forcing their way through to the staircase before climbing to her apartment.

It was the first time in more than two years that friends have been able to see Liu, who has not been charged with any offence.

She appears emotionally shaken by the visit and is seen asking the visitors to leave, apparently out of fear of retribution from the authorities.

"You have to go, or they will come and bring trouble," she says at one point.

Mostly she whispers in the ear of one of the activists so that the camera will not pick up her words.

In between sobs and smiles, the rights campaigners -- including Hu and respected academics Xu Youyu and Hao Jian -- agree after a brief stay to leave.

All three signed Charter 08, a bold petition for the protection of human rights issued in 2008.

"Let's be quick, we have to leave before they come up," says one, as pounding on the door signals the arrival of security guards.

The visit took place on Friday, Liu Xiaobo's birthday.

Hu said in a posting under the video that a fourth activist, Liu Di, was also present. She is not related to Liu Xia.

He added: "The thousands of words we wanted to say all turned into tears and choked cries."

He later told AFP that after leaving the apartment the group were detained momentarily by the guards but eventually allowed to leave.

"The video is all about fear and anxiety," Hu said. "She has already lost a lot of hope. The authorities are making her fearful. What she is afraid of is her family will come under pressure.

"Imagine people come to visit you after two years under illegal house arrest and all she feels is fear that the authorities will crack down further."

Liu Xiaobo was arrested and sentenced to 11 years in prison for subversion after spearheading Charter 08.

Chinese authorities tried to block news of his Nobel award in 2010 and put his wife under house arrest.

The activists who visited Liu Xia have long sought political reform and greater respect for human rights in the one-party state.

"For two years, they have only allowed Liu Xia to visit her parents and go see Liu Xiaobo, no one else. She is like a political prisoner, when she meets her husband it is a meeting between prisoners," Hu said.

"I went to visit her as friend, I have the right to visit her. The authorities know who I am, I have done nothing wrong. Her house arrest is illegal."

Meanwhile, a group of Chinese intellectuals has called on the government to implement urgent political reforms, respect human rights and rule the nation in accordance with the law or risk "violent revolution".

In an open letter 71 top academics warned that growing economic imbalances were fuelling social unrest and an uprising could erupt if reforms were not implemented immediately, Hu Xingdou, one of the signatories, told AFP Monday.

"If urgent systematic reforms needed by Chinese society continue to suffer setbacks and stagnate, then official corruption and social dissatisfaction will boil up to a crisis point," said the letter, posted on the Internet last week.

"China will once again miss the opportunity for peaceful reform, and slip into the turbulence and chaos of violent revolution."