Authorities in the southwestern Chinese province of Sichuan have begun holding activists and petitioners under tight surveillance and detention ahead of a sensitive political anniversary and a planned protest against a petrochemical plant on Saturday. Activists had planned to take to the streets on Saturday in protest at a newly built paraxylene (PX) plant in the Pengzhou suburb of the provincial capital Chengdu, using the anniversary of May 4 student demonstrations in 1919 and the fifth anniversary of a local movement against the plant. The plant, owned by state-run PetroChina, has been halted amid safety concerns in the wake of last week's magnitude-7 earthquake near Sichuan's Ya'an city that left more than 200 people dead or missing and reignited concerns about the plant's potential health hazards. "They were talking online about a walk-past in Jiuyan Bridge in Chengdu tomorrow, as a protest," said Chengdu-based activist Chen Qian, who was prevented from leaving her home late on Thursday by neighborhood committee officials, who said the house arrest would likely last until Monday. "As I was leaving the building to go out, they said they weren't to let me leave home, and that's the way it had to be," she said. "They said it would be two or three days," Chen said. "I think it's because of tomorrow." Local residents had protested against the chemical plant at the bridge in 2008 on May 4, a date that has been observed as an occasion to call for freedom and social change in the spirit of the 1919 movement when students championed "democracy" and "science" as forces to modernize China. Following a devastating quake that hit the province later that month, the government of Chengdu, which is home to more than 14 million people, promised to re-assess the environmental impact of the 38.1 billion yuan (U.S. $6 billion) plant, but construction began on the facility in 2011. 'On holiday' In Chengdu's Wenjiang district, activist Jiang Yuqiong said she and her husband had been taken away "on holiday" by officials to Black Dragon Lake near Meishan city. "[We will be back on] Sunday," Jiang said. The Chengdu-based rights website Tianwang said that Zhou Wenming, an activist from Sichuan's Shuangliu county, had been taken "on holiday" along with fellow activists Zhao Xianqiong and Yang Fang. Chengdu activist Xin Wenrong said he was summoned by the local branch of the state security police after a friend forwarded a post protesting the Pengzhou petrochemical plant to him, dating from 2010. "They used an illegal procedure for the summons," Xin said. "Some petitioners have been 'taken on holiday' today because of the Pengzhou petrochemical plant issue." Schools remaining open He said primary school and high school students in Chengdu had been forced to attend class all weekend, in a bid to stop the demonstration going ahead. "I tried to get onto[Twitter-like services] Sina and Tencent Weibo, to search for information, but Pengzhou petrochemical is already a sensitive word," Xin said. "If the government is sincere about communicating with the people, they should use methods other than detaining and threatening them, or forbidding print shops to photocopy certain things, and stop pretending there is no such thing as dissent," Xin said. Online censorship A 33-year-old woman was arrested on Friday after she called via her microblog account for a protest on May 4 against the plant, the Hong Kong English-language South China Morning Post reported. "In a post on Thursday on her microblog that has since been deleted, she also said the protest had been approved by authorities," the paper said. The security clampdown appeared to extend further than Chengdu, however, with online censorship blocking information about the protest, and tight security in Beijing. Search terms linked to the planned protest were blocked on social media sites on Thursday, including searches in Chinese for "Chengdu PX project," "May 4th+Jiuyan Bridge+take a walk," "Pengzhou+PX," and "Pengzhou+petrochemicals," according to the China Digital Times website, which monitors censorship edicts from Beijing. Surveillance in Beijing Authorities in Beijing stepped up surveillance of activists and petitioners, ordinary Chinese who pursue official complaints against the government in the capital, ahead of the sensitive May Fourth Movement anniversary on Saturday. Liaoning petitioner Zhao Guangjun said Tiananmen Square was basically sealed off on Friday, suggesting that the ruling Chinese Communist Party may have some activities of their own planned there. "They will be watching the university campuses tomorrow, as well as the embassy district and Tiananmen Square," Zhao said. "Some people are sure to get arrested tomorrow, or sent back home, or locked up," he said. "All of those things will likely happen." Jilin petitioner Deng Zhibo said many petitioners still planned to pursue their complaints outside central government offices on Saturday, however. "The more commemorative or sensitive a day is, the more it will attract petitioners," Deng said. "The petitioners aren't afraid of being sent home or locked up."