A Vietnamese engineer imprisoned for his activism has been beaten, humiliated and treated like a "slave" in jail after refusing to wear prisoner uniform, his brother and a former inmate said, calling for pressure on the authorities to bring him out of "hell." Dang Xuan Dieu, jailed for 13 years in 2013 on charges of plotting to overthrow the authoritarian government in Hanoi, has also been refused family visits after he sent a letter to the police minister complaining about the mistreatment. "They treated him very badly," Catholic activist Dieu's brother Dang Xuan Ha told RFA's Vietnamese Service. "Dieu said he is innocent so he did not wear the uniform bearing the word "criminal," Ha said. "Dieu protested the fact that his letters sent to the authorities, including the police minister, have not been answered. That was why they did not let him meet his family." Dieu got the biggest jail term among a group of Catholic activists, students, and bloggers convicted for their involvement with Viet Tan, a U.S.-based Vietnamese political group outlawed and considered a terrorist organization in Vietnam. Dieu was incarcerated in No. B4 prison in Hanoi, but was later moved to No. 5 prison in Thanh Hoa province. Prison authorities allowed Dieu's family to visit him only once while the he was in the Hanoi prison, but have not permitted them to see him in the other detention center, Ha said. Truong Minh Tam, a former prisoner who was confined in a cell next to Dieu's told RFA that prison staff humiliated him for several months last year by letting other prisoners beat him and forcing him to serve as a "slave." "He was living in hell because they[prison staff] humiliated him," Tam said. Dieu was not given access to a fan or clean drinking water, said Tam, who had served one-year jail term after he participated in protests against China whose territorial disputes with Vietnam have led to riots and a sharp deterioration in bilateral relations. Prison authorities also forced Dieu to pose as a "model" for other prisoners who were asked to paint him as a "half-human/half-beast figure," Tam said. Dieu had also staged a hunger strike in a campaign for prisoners' rights, Truong said. Dieu's family said that news of his hunger strike in June had been mentioned in petitions sent to foreign embassies in Vietnam and to the U.N. Special Rapporteur on Freedom of Religion or Belief. "I would like people from different religions backgrounds to raise their voices in pressing the government to stop the ill treatment of Dieu," Tam said. He added that Dieu's 70-year-old mother was not in good health and dispirited by her son's incarceration. Relatives of Dieu and the other jailed activists had staged a protest march and candlelight vigil outside government offices in 2012 after they were first detained a year earlier. They wanted to submit a petition demanding the activists' releases, but were blocked by police.