BEIJING >> Chinese security forces rounded up more government critics ahead of Saturday’s anniversary of the crushing of the 1989 pro-democracy movement centered on Beijing’s Tiananmen Square, adding to an already harsh crackdown on dissent, activists said.

The Foreign Ministry, meanwhile, lashed out at the U.S. government over calls for a full accounting of the military assault on civilians 22 years ago, saying the issue was closed.

"A clear conclusion has already been made concerning the political turmoil that happened in the late 1980s," spokesman Hong Lei was quoted as saying by the official Xinhua News Agency.

Stricter measures against dissidents are routine on the June 4 anniversary, but this year coincide with the most sweeping suppression campaign in many years. Hundreds of activists, lawyers and bloggers have been questioned, detained or simply have disappeared in the four-month campaign that aims to quash even the possibility of a pro-democracy movement forming along the lines of those sweeping the Arab world.

Bao Tong, a former aide to the late liberal Communist Party Secretary Zhao Ziyang, was taken to an unknown location by security officers this week along with his wife, according to Chinese Human Rights Defenders, a group that publicizes information on dissidents collected from sources within China.

Bao served a prison sentence following the military crackdown, while Zhao, his former boss, was deposed for sympathizing with the protesters and lived out his life under house arrest in Beijing. Calls to Bao’s home rang unanswered Saturday.

Ding Zilin, who founded a group for people whose children were killed in the crackdown, was placed under house arrest while a number of former activists in the student-led protest movement were taken from their homes or told not to go out, the group said.

Chen Ziming, whose liberal think tank sought to mediate between the students and Communist Party leaders, was told he would not be permitted to leave home before June 10, the Hong Kong-based Information Center for Human Rights and Democracy said.

A number of other activists have been warned not to leave home, issue statements, or speak to media, according to the two groups.

As usual, the anniversary was ignored by China’s entirely state-controlled media while Tiananmen Square in the heart of the capital was open under heavy security.

Twenty-two years later, few young Chinese remember the events that marked the last popular challenge to Communist rule in the country. The decades since have seen the economy boom and the Communist Party relinquish much of its day-to-day control over many areas of society while still making no significant moves toward changing the one-party authoritarian political system.

The Chinese government has never fully disclosed what happened when the military crushed the weekslong Tiananmen protests, which it branded a "counterrevolutionary riot." Hundreds, possibly more, were killed when troops backed with tanks fought their way to the square into central Beijing on the night of June 3-4.

In Hong Kong, which returned to Chinese rule in 1997 but maintains its own British-style legal system, tens of thousands of people held aloft candles to mark the anniversary in a large park, turning six soccer fields into a sea of light.

Democracy activists laid a wreath at a makeshift memorial and bowed three times in customary Chinese mourning tradition. Crowds watched video messages from Ding Zilin and Wang Dan, one of the 1989 movement’s leading voices.

"We want to give a very strong message to the Communist regime that they cannot suppress the memory of June 4," said Lee Cheuk-yan, a lawmaker and pro-democracy activist. "For China, it is the darkest age for human rights. We can see all the human rights defenders being arrested and suppressed."

In Taiwan, the self-governing island democracy that China claims as its own territory, President Ma Ying-jeou issued a statement calling on China to respect human rights, institute political reforms and release imprisoned dissidents.

Beijing usually avoids discussing the crackdown and the Foreign Ministry’s statement displayed the government’s growing tendency to confront Washington over sensitive issues. Hong referred to recent remarks by State Department spokesman Mark Toner urging China to account for those killed, injured and detained in the crackdown, along with a similar call from House Minority Leader Nancy Pelosi.

"The U.S. statement, which ignored facts and groundlessly accused the Chinese government, is a rude interference in China’s internal affairs and its judicial sovereignty," Hong was quoted as saying.

He said China’s people enjoy extensive rights and freedom, and the country’s soaring economic and social development over the past two decades attest to the government’s correct leadership.

"We urge the U.S. side to abandon its political bias and rectify the wrong practice to avoid disturbing the China-U.S. relations," he said.

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Associated Press writer Kelvin Chan in Hong Kong contributed to this report.