Authorities in the central Chinese province of Hunan questioned two activists who tweeted in support of hunger-striking political prisoner Guo Feixiong, they told RFA.
Guo, whose birth name is Yang Maodong, has been subjected to forced feeding after beginning his hunger strike in early May in protest at the treatment of political prisoners in China, his sister Yang Maoping has said.
"Today is the 100th day of my brother's hunger strike," Yang told RFA on Tuesday.
"I am still very worried about the state of his health," she said, but declined to comment further.
Meanwhile, Hunan-based activist Zhu Chengzhi and Zhou Jie, both close friends and supporters of Guo, were questioned by state security police in their hometowns after they posted photos on social media sites marking the 100th day of his hunger strike.
Zhu, Zhou and two other activists had unfurled a banner proclaiming support for Guo at the popular mountain tourist destination of Yuelu Shan near the provincial capital Changsha on Saturday.
"This is the 100th day of his hunger strike!" the banner proclaimed. The activists then posted photos of the banner to social media accounts.
Zhu and Zhou received calls from the Hunan state security police on Tuesday.
A third activist who took part in the Yuelu Shan protest, known by his nickname Tiezi, was also summoned for questioning by his local police station, Zhu and Zhou said.
"We just wanted to do something to show our support," Zhou told RFA. "There is very little else we can do."
"The authorities should respond to people's concerns about his health; he is in a poor state, and we are afraid he can't take [the hunger strike] anymore," he said.
Threats to use electric batons
London-based Amnesty International recently quoted sources as saying that guards at Yangchun Prison, where Guo is being held in the southern province of Guangdong, have threatened him a number of times with the use of electric batons if he doesn't do as they tell him.
The authorities have also refused Guo's request to be transferred to a different jail, the group said.
It repeated calls for Guo's immediate release, citing his rapidly decreasing body weight.
Zhu said the threats were unlikely to be enough to suppress Guo, or other activists, however.
"From the point of view of Guo Feixiong and many others who dare to stand up in protest, the threat of electric shocks ... won't stop them," he said.
Relay hunger strikes
Guo began his hunger strike calling on President Xi Jinping to implement democratic reforms, end the use of electric shocks in prison, improve the treatment of political prisoners, and ratify a United Nations covenant on civil and political rights.
More than 400 rights activists have been on relay hunger strikes in support of Guo since he began refusing food and water.
According to the New York-based Human Rights Watch (HRW), Yangchun Prison twice admitted him to hospital between April and May, but only for check-ups. No diagnosis or medical treatments were offered.
Guo began his hunger strike on May 9 after being subjected to a forced rectal cavity search at the instigation of state security police, as well as forced head shaving and verbal abuse from prison guards.
Guo was sentenced last November for "picking quarrels and stirring up trouble" and "gathering a crowd to disrupt social order" after a prolonged period in pretrial detention.
During his sentencing hearing, Guo shouted in protest at his treatment while in police custody, where he was held in solitary confinement in a small, dark cell and denied permission to exercise outdoors since August 2013.
Reported by Hai Nan for RFA's Cantonese Service. Translated and written in English by Luisetta Mudie.