In the letter, the 109 signatories say the system is inconsistent with the rule of law because police can send the women to detention centers without the involvement of prosecutors or courts.
The administrative regulations governing the system —which is supposed to give sex workers “legal and moral education” — are not backed by law, it said.
The move comes amid stated attempts by the Communist Party to make the judicial system fairer.
One of the signatories, Liu Jianshu, said the letter had been sent by courier to the National People’s Congress Standing Committee on Monday.