Chinese lawyers have urged Twitter to fight for the release of a woman sent to a labour camp for a sarcastic tweet.

Human rights activist Cheng Jianping, known to friends as Wang Yi, was sentenced in November to a year's re-education through labour. She had retweeted her fiancee's satirical suggestion that anti-Japanese protesters should attack Japan's pavilion at the Shanghai Expo, adding: "Angry youth, charge!"

Cheng's fiance said they had been angry at anti-Japanese sentiment.

Her lawyers have been unable to meet her since she was sent to the re-education centre in central Henan province.

In a letter to Twitter's chief executive officer, Dick Costolo, published on the website of the group Human Rights in China, Cheng's lawyers said they were "encouraged and gratified from the bottom of their hearts" when they saw that he had tweeted about the detention.

Costolo wrote: "Dear Chinese government, year-long detentions for sending a sarcastic tweet are neither the way forward nor the future of your great people."

Teng Biao and Lan Zhixue said in their letter that the case set a precedent because Cheng, 46, was the first mainland Chinese Twitter user to be criminally punished for sending a normal tweet.

They wrote that the decision amounted to infringement of Twitter's lawful business activities and urged Twitter to take legal action to protect its interests – although it is not clear how it could do so.

They also called on Twitter to use its "international influence" to press for the unconditional release of Cheng, pointing out that world leaders such as Barack Obama and the Dalai Lama have accounts.

Teng told the Guardian that although Twitter was blocked in China it was "very important … and getting more and more important".

"The number of users is not huge compared with the Chinese population, just a bit more than 100,000, but this group includes people who are crucial to bring changes, such as journalists, lawyers and intellectuals.

"Wang Yi's case has certainly had an effect … People may not be using their real names on Twitter but it is very easy for the government to find who they really are. This will make people more cautious about what they say."

Domestic microblog services have proved hugely popular in China, with an estimated 125 million users by the end of last October. But they are heavily censored.

Cheng's fiance, Hua Chunhui, told Associated Press after her arrest: "My personal opinion is that this sentencing wasn't about this one statement. The government wants to make an example of us activists … We actively communicate with other Chinese activists and celebrated on Twitter Liu Xiaobo's Nobel [peace] prize."