Authorities in the northeastern Chinese province of Heilongjiang have detained four prominent rights lawyers representing detainees in an unofficial detention center, or "black jail," fellow lawyers said on Friday. Jiang Tianyong, Tang Jitian, Wang Cheng, and Zhang Junjie were detained by authorities on Friday morning at their hotel in Heilongjiang's Jiamusi city, Guangzhou-based rights lawyer Li Xiaoling told RFA's Mandarin Service. The lawyers had been hired by relatives of those held in the "black jail" to campaign for their release, according to Beijing-based rights lawyer Chen Jiangang. "These four lawyers were representing human rights cases on the front line," Chen said. "They detained them and confiscated their cell phones." "I have tried to call them several times, but no one is picking up, and I can't get through; there's a switched-off message," he said. Fellow rights lawyer Zhao Yonglin confirmed the report. "The situation now is that the four lawyers have been locked up in a detention center, on the grounds that Wang Cheng's lawyer's license is fake," Zhao said. Threatening call Zhao said he had received a call from a number listed as belonging to the Jiamusi police chief, threatening that if the black jail case wasn't dropped, the lawyers "would find it hard to practice their profession." But the caller, who had a northeastern accent, declined to give his name or job title, he said. Zhao said the lawyers had taken issue with an extrajudicial detention center officially known as a "legal study center," where the authorities send anyone who pursues complaints against them, particularly with higher levels of government. "There's a place[in Qinglongshan] called the legal study center, where they hold citizens prisoner who haven't committed any crime," Zhao said. "The four lawyers were detained while on an inspection visit to the Qinglongshan black jail ... the sign on the gate reads Jiansanjiang Rule of Law Education Center," said Zhao, who had planned to join the other four lawyers, but canceled owing to other commitments. "In fact, it's a black jail." He said many of the detainees are being held there for religious reasons. "They have a lot of groups with religious beliefs locked up in there under the guise of legal education," Zhao said, adding, "They hold them for several months at a time, even up to a year." Hired by relatives He said the four lawyers had been hired by relatives of detainees to get them out. "They were there to demand their release, because they were locked up without any legal process at all," Zhao said. Rights lawyer Chen Jiangang said China's embattled legal profession has been under increasing pressure in recent years. "There are no human rights in China, and we lawyers have to try to do our jobs in manacles," Chen said. "It's a humiliating business, being a lawyer in China." "There's no guarantee of one's personal rights or safety, and yet we still try to help others defend their rights," he said. "We are pretty helpless." China's parliament voted on Dec. 28, 2013, to end its controversial "re-education through labor," or laojiao, system of administrative punishments following a prolonged campaign by lawyers, former inmates, and rights activists to abolish it. But rights lawyers say many other forms of extrajudicial detention remain, and that some former laojiao camps have changed their name but not their function. Torture, abuse Rights activists in China have long campaigned for the abolition of "black jails," which are often used to detain those who complain to higher levels of government about local officials or to hold anyone regarded as a troublemaker or threat to "social stability." Those held without due legal process are at increased risk of torture and general abuse, rights groups say. Staff in such detention facilities often insult and humiliate detainees, and have even robbed, raped, seriously injured, and killed them, according to a report last year by the Chinese rights website Weiquanwang. In May, Sichuan authorities detained and beat high-profile rights lawyers who tried to visit an unofficial detention center, or "black jail," according to fellow lawyers who spoke with them during the attack. And according to London-based rights group Amnesty International, thousands of Chinese people are subjected to arbitrary detention in labor camps and unofficial "black jails" each year. Earlier this month, Beijing announced a rise in the domestic security, or "stability maintenance," budget to 205 billion yuan (U.S. $33 billion). UPDATE 27/03/2014: Zhang has been released, but Tang, Jiang, and Wang are serving 15-day administrative sentences at the Qixing Detention Center in the city of Jiansanjiang. Tang has been charged with "using a cult to disrupt social order," but charges on the others remain unknown. Unconfirmed report allege that they have been subjected to torture. Five activists campaigning for their release were detained and released on 26 March. UPDATE 02/04/14: The reasons for the lawyers' detention have become clearer as authorities have claimed that they were visiting detained members of Falun Gong. Global Times newspaper, a CCP party mouthpiece, accused the lawyers of "instigating confrontation" and "inciting illegal activity" through their visit. Zhang -the only of the four who has been freed- has claimed that he was subjected to severe beatings which resulted in three cracked ribs. UPDATE 06/04/14: The three remaining lawyers-Tang Jitian, Wang Cheng, and Jiang Tianyong- were released. All four of the lawyers were subjected to torture while in detention: they suffer from lost teeth, broken ribs and soft tissue injuries.