The bold plea for political reform calls on the government to allow freedom of expression and association and to give up Communist Party control over the legal system.

Its launch coincides with Wednesday's 60th anniversary of the Universal Declaration of Human Rights and is one of the most dramatic steps taken by dissidents since a crack down during the run-up to the Beijing Olympics.

The authorities have already moved against those responsible, an informal group headed by lawyers and journalists, according to some of the more than 300 signatories to the document.

Several have been detained for questioning, and one of China's most prominent dissidents, Liu Xiaobo, who was imprisoned after the 1989 Tiananmen Square massacre, was still being held on Tuesday night.

"After we handed over the Charter, I was detained by the police," said Zhang Zuhua, a writer and political analyst. "Right now the police have searched my home. My computer and documents have all been taken away.

"I am now being held at my home. I can't go anywhere. I don't know what will happen to me next. But I was told by the police that they are carrying out a formal investigation, so I think there will soon be further punishment."

Other signatories have been detained in Zhejiang province in central China, and Guizhou in the south.

The original Charter 77 was a declaration by dissidents in Soviet-era Czechoslovakia which called upon the then Communist government to ratify United Nations human rights conventions, and honour the human rights obligations of the Helsinki Accords of two years earlier.

Among the signatories to that document were the future Czech president Vaclav Havel, and other leading writers.

As with the original, many of those behind Charter 08 are keen to work within the framework of the Chinese constitution. They call on the government to ratify the International Convention on Civil and Political Rights, a document which Beijing has signed but not put into practice.

Among their most politically sensitive demands are for western-style separation of powers - the legislature, the executive and the legal system - which has been publicly discussed but ruled out by Party leaders, the establishment of a human rights commission and direct elections for a choice of political parties.

The signatories include well-known dissidents and critics of the government such as Mr Liu, and Professor Ding Zilin, who heads the Mothers of Tiananmen campaign which publicises the deaths of those such as her son who died in the military suppression of pro-democracy demonstrations in 1989.

Others are prominent academics at leading universities and well-established lawyers.

The Chinese Human Rights Defenders, another informal group of activists, said it had even won the backing of government officials.

"This charter promotes the same ideas and values that the universal declaration of human rights asserts," said Mo Shaoping, a lawyer.

"It has nothing that goes against China's constitution."

The government has so far made no response. But state media carried interviews with senior officials to mark the Declaration's anniversary acknowledging a string of problems with human rights in the country, though they also stressed the progress China has made in the last 30 years.

The Chinese Human Rights Defenders said the Charter "outlines a vision for a constitutional democratic China and initiates a platform for citizens' actions".

Its statement condemned the arrests of signatories.

"The crackdown on human rights activists, being directly related to the anniversary, demonstrates contempt towards international human rights norms and insincerity in the government's own pledges and commitments to promote human rights in China," it said.