It is the second time in three months that such demands have been made, the BBC reported.
The letter, posted on several Chinese websites and blogs, appeared just ahead of next month's annual Chinese parliamentary session where the country's new Communist Party leader, Xi Jinping, will also be named president.
It urged ratification of the international human rights treaty, the BBC reported.
"We solemnly and openly propose the following as citizens of China, that the International Covenant on Civil and Political Rights ... be ratified, in order to further promote and establish the principles of human rights and constitutionalism in China," the letter said. China signed the document in 1998 but it has yet to ratify it.
Xi, known as a reformer, was installed as party leader at last November's Party Congress, which also marked the country's once-in-a-decade leadership transition. In March, he will also be made president, succeeding Hu Jintao.
The report said those signing the open letter included economist Mao Yushi, legal scholar He Weifang and outspoken political activist Dai Qing. A similar letter in December also demanded political reform including an independent judiciary and meaningful democratic change, the report said.
The December letter had warned that without the needed reforms, China's official corruption and dissatisfaction in society would lead to turbulence and chaos.
The BBC report said the language in the latest letter was conciliatory as it noted difficulties in bringing about meaningful political change.