Authorities in China on Monday stepped up a nationwide security operation targeting critics of the ruling Chinese Communist Party on the anniversary of a 1989 massacre that ended weeks of student-led protests on Beijing's Tiananmen Square. Zhang Xianling, a founder member of the Tiananmen Mothers victims group who lost her 19-year-old son during the crackdown, said she is currently under closer surveillance than in previous years, after the group wrote to Chinese President Xi Jinping calling the bloodshed a "crime against humanity." "There have been a lot of police following us closely," Zhang, who went to pay her respects at her son's grave this week, said. "There were also a lot of police at Wan'an Cemetery; several dozen of them, all in plain clothes." In the 29 years since the massacre, the Tiananmen Mothers victims' group has called repeatedly for a public inquiry into the massacre. They also want compensation, and a detailed account of who died, how and where, but the government has never responded. Jiang Jiawen, a rights activist from northeastern China currently living in Beijing, said police are also out in force along all major public transportation routes leading to Tiananmen Square. "In Beijing, particularly to the east of Tiananmen Square, they are checking ID cards wherever you go, on such a sensitive day," Jiang said. "Plainclothes state security police are checking the ID of anyone they think looks like a dissident or a petitioner." "They are doing this on the buses, too, and there are police and security guards checking people's ID on the subway," he said. Jiang said he and fellow rights activist Tian Ye had been detained for making posts to social media about the 29th anniversary of the crackdown, when the People's Liberation Army (PLA) used armored vehicles and machine guns to put an end to weeks of student-led popular protests calling for an end to official corruption, and political reform. "We were taken away by a high-ranking officer from the Fangshan police station, as well as four officials from our hometowns' representative offices in Beijing," Jiang said. "We are now staying in a guesthouse run by the representative office." "They always put me under surveillance at this sensitive time, because it's June 4, and we were writing about it online," he said. "Most of the key figures[who are trying to commemorate the crackdown] across China have now been placed under surveillance or house arrest now." Sources said petitioners Zhu Xiaoping, He Bin and Xu Caihong were also detained by police on Tiananmen Square on Monday.