The apparent suicide of the father of a Chinese dissident is being disputed by his loved ones, after authorities reported he jumped from a building after being detained, a rights group said.

Xue Fushun was the father of activist Xue Mingkai, who spent four years in jail after joining a banned political party and taking part in pro-democracy activities. He was freed last September.

According to a statement released by Human Rights in China Monday, and confirmed by a family friend, both Xue's father and mother, Wang Shuqing, were taken away by police in Qufu in eastern Shandong province on January 23.

Six days later they fled to a government public prosecutor's office but were soon caught. After being split up, Wang was told her husband had jumped to his death from the prosecutor's office, the rights group said, citing "informed" sources.

"Of course it's suspicious," said family friend Ma Qiang, confirming the rights group's account. "This situation of 'being suicided' is very common in China."

The couple were taken into custody "many times a year and he had always said: 'I would never commit suicide'", Ma told AFP by phone.

Supporters of the family have signed an open letter urging an independent inquiry, which was posted online by Human Rights in China.

"Xue Fushun's death is not an isolated incident," the statement said. "Dissidents are in fact not protected by the law."

Authorities "can violate their human and civil rights at will, even going as far as persecuting their family members", it added.

Wang, who remains in custody, was only allowed to see the head and neck of her husband's corpse, said Ma.

Ma added that he had spoken to Wang in recent days and helped her escape detention briefly.

Xue Mingkai, who is in his mid-20s, has not been brought in, Ma said.

Xue was first convicted in 2010 for "state subversion" after joining the banned China Democracy Party, and again in 2012 for "inciting state subversion" for his continued activism.

Beijing takes a hard line on dissent, and after outlawing the pro-democracy party, it jailed numerous organisers.

Leading dissident Li Wangyang, jailed for more than 22 years after the 1989 Tiananmen democracy protests, was found hanged in a hospital room in central China in 2012, with activists accusing local officials of foul play.

Over the past year authorities have detained at least 20 activists from a burgeoning New Citizens rights movement after they organised small-scale protests for a variety of causes from anti-corruption to educational equality.

So far 10 members have been charged with disrupting public disorder. In recent weeks three in Beijing have been found guilty and sentenced to up to four years in prison.