Ten Chinese lawyers have written an open letter urging the government to reform its system of hard labour after a mother was sent to a camp for challenging the sentences of men who raped her daughter.

The letter was reported on the website of state newspaper the People's Daily and calls for a more transparent hearing process for so-called "re-education through labour", which critics say is often used to punish dissent.

"It's a kind of privilege for the police and has been constantly abused," signatory Liu Weiguo said of the system. "It violates the constitution and human rights."

The lawyers said they wanted to take advantage of recent public outrage over Tang Hui, who was sentenced to 18 months' hard labour for protesting over the sentences handed to seven men who abducted and raped her 11-year-old daughter.

Tang was released last Friday after her lawyer appealed the sentence and authorities have said they will investigate her grievances, but her heavily publicised case led to demands that China's labour camps be scrapped.

Rights lawyer Li Fangping, who came up with the idea of writing the letter, said its publication on the website of the People's Daily website suggested some within the Communist party agreed the system needed to be changed.

Li said he would like to see it abolished altogether, but that with a handover of power due to begin later this year, reform was a more realistic aim.

The National People's Congress -- China's parliament -- had floated reforms as early as 2005 but had not made progress, he said.

"We will call for more public attention, then some regulations should come out. It's a long process," he told AFP by telephone.

Reeducation through labour was introduced in 1957 to punish minor offences, and has since netted a wide range of citizens, from drug offenders to petitioners to members of outlawed religious activities.

More than 400,000 people were sent to labour camps in the first 50 years, the state-run China Daily has reported.

Sentences of up to four years are issued by administrative rather than judicial panels, and appeals rarely succeed, according to the China-focused rights group Dui Hua.

The lawyers in their letter called for a more systematic hearing process, public records of verdicts and the names of those who issued them, private lawyer-client meetings and an age limit of 60 for those liable to the sentence.