BEIJING — Beijing put additional police on the streets and detained government critics Tuesday as part of a security crackdown on the eve of the 25th anniversary of the crushing of prodemocracy protests centered on the capital’s Tiananmen Square.
Police manned checkpoints as officers and paramilitary troops patrolled pedestrian overpasses and streets surrounding the square.
The increased security comes on top of heightened restrictions on political activists, artists, lawyers, and other government critics. Dozens have been taken into detention, forced out of Beijing, or confined to their homes in other parts of the country.
‘‘June 4 has come again and the plainclothes officers are here to protect us. I can’t leave the house to travel or lecture,’’ Jiangsu province-based environmental activist Wu Lihong said in a text message.
Artist and former activist Guo Jian was also taken away by authorities on Sunday night, shortly after a profile of him appeared in the Financial Times newspaper in commemoration of the crackdown’s anniversary. As he was being detained, Guo, an Australian citizen, told an Associated Press reporter he would be held until June 15.
A writer and officer of the Independent Chinese PEN Center, who writes under the name Ye Du, was also taken from his home in the southern city of Guangzhou to join a forced ‘‘tour trip,’’ his wife, Wan Haitao, said by phone. Such compulsory trips are a common method of keeping government critics under 24-hour watch without the need to initiate legal proceedings.
In an apparent sign of government nervousness, connections to the global Internet appeared to have been disrupted, with Google’s mail and other services mostly inaccessible. China already routinely blocks popular overseas social media sites such as Twitter and YouTube and heavily censors Chinese sites for politically sensitive content.
China allows no public discussion of the events of June 3-4, 1989, when soldiers accompanied by tanks and armored personnel carriers fought their way into the heart of the city, killing hundreds of unarmed protesters and onlookers. The government has never issued a complete, formal accounting of the crackdown and the number of casualties.
Beijing’s official verdict is that the student-led protests aimed to topple the ruling Communist Party and plunge China into chaos.
Protest leaders said they were merely seeking greater democracy and freedom, along with an end to corruption and favoritism within the party.
Asked about the crackdown at a regularly scheduled news conference, Foreign Ministry spokesman Hong Lei did not refer directly to Tiananmen Square or the military crackdown.
‘‘Regarding the political incident which happened in the late 1980s in China, as well as issues related to it, the Chinese government reached a conclusion a long time ago,’’ Hong said before launching into a defense of China’s economic reforms that have created a burgeoning middle class amid relative political stability.
Hong also denied cases of political persecution, saying: ‘‘In China, there are only law offenders. The so-called dissidents as you mentioned do not exist.’’
Authorities regularly tighten security ahead of June 4, but this year’s suppression is notably harsher than in the past. Activists who previously received no more than a warning have been taken into custody and police have told foreign journalists they would face unspecified serious consequences for covering sensitive issues ahead of the anniversary.
‘‘I urge the Chinese authorities to immediately release those detained for the exercise of their human right to freedom of expression,’’ UN Human Rights Commissioner Navi Pillay said in a statement Tuesday. She also criticized authorities’ restrictions on social media and the press for the anniversary.
Pillay called for a truth-seeking process into the events a quarter-century ago. ‘‘It is in the interests of everyone to finally establish the facts surrounding the Tiananmen Square incidents,’’ she said.