The National Union of Journalists said it had sent a letter on Friday to Dominique Vidal, Yahoo! Europe's vice-president, protesting the internet company's alleged actions in China.
Chinese courts have said that Yahoo! supplied the authorities with information that helped them identify, prosecute and jail writers advocating democracy.
"The NUJ regards Yahoo!'s actions as a completely unacceptable endorsement of the Chinese authorities," wrote Jemima Kiss, chairman of the NUJ new media council, in the letter to Vidal.
Mary Osako, a Yahoo! spokesperson, said the US-based company believes it must conduct business in each country in ways that comply with local laws.
"Let us make clear that we condemn punishment of any activity internationally recognised as free expression, whether that punishment takes place in China or anywhere else in the world," Osako said.
Terry Semel, Yahoo!'s chairman and chief executive, said at the company's annual shareholder meeting last week that the company had no choice but to comply with local laws and did not have the power to change Chinese policy alone.
Yahoo! has been accused by the NUJ and other journalism groups of providing information that led to an eight-year prison term for Li Zhi for discussing pro-democracy issues in a web forum.
The company has also been reported as helping to identify Shi Tao, who was sentenced to prison for 10 years for forwarding a government email to the foreign press.
The NUJ also accuses Yahoo! of providing records that led to the imprisonment of Jiang Lijun - who was sentenced to four years in jail for writing articles advocating democracy.