Correction: An earlier version of this article said it has been 25 years since the Tiananmen Square protests. It has been 24.

Democracy activist Hu Jia said he spent Tuesday alone in a hotel room in southern China, where he has been confined for the past week under the watchful eyes of eight to nine plainclothes security agents.

The restrictions on him and other dissidents have become an annual routine on June 4 — the anniversary of the government’s violent suppression of the 1989 protests in and around Tiananmen Square — when much of the country is placed on a lockdown of sorts by the ruling Communist Party.

On Tuesday, China’s restricted Internet was more finely edited than usual, with censors deleting posts that contained even the vaguest reference to that day 24 years ago when the government forces opened fire on unarmed civilians. The square itself was flooded with uniformed and plainclothes police.

The heaviest surveillance, however, was reserved for dissidents such as Hu, who are often put under house arrest on the anniversary or taken by police to provinces far from Beijing.

The annual ritual of hypervigilance and attempts to evade it has evolved into a kind of cat-and-mouse game. Bloggers use increasingly creative means of outflanking the censors. And dissidents form elaborate plans weeks in advance to ensure that their messages can be circulated if their communications are cut off.

Profiles: Tiananmen witnesses on what they tell their children.

For all the absurdities that can result, activists say their efforts at least help keep alive the memory of those who died during the student-led protests against government repression and corruption.

In Hu’s case, he had continuous access to the Internet though he was confined to the hotel room. So he spent Monday and Tuesday promoting a campaign for citizens to wear black T-shirts in solidarity with the dead.

It was an idea he had incubated for months with other activists, he said in a phone interview Tuesday from the hotel in Guangzhou. “The authorities can’t do anything about T-shirts. There is no law forbidding wearing them,” he said.

Some people posted pictures of themselves in black shirts online, but it was impossible to gauge the popularity of the campaign because censors took down many of the images.

Others deployed different tricks online. With the date “June 4” and its numerical equivalents — “64,” “six 4” and “6 four” — censored on microblogs, some used “0.8*8” as a workaround for “6.4.”

One particularly widespread post on weibo, Chinese microblogging Web sites similar to Twitter, was a doctored version of the iconic photo of a man standing defiantly in front of Chinese army tanks. In the photo, the tanks were replaced with giant yellow rubber ducks, a joking reference to a recently popular inflatable art piece in Hong Kong’s harbor.

In response, the phrase “big yellow duck” was quickly censored, along with pictures of lighted candles, the word “today” and a Lego variant of the tank-man picture.

For a few days leading up to Tuesday, weibo seemed to be experimenting with a more sophisticated way of censoring searches. Instead of simply blocking queries on banned words, users were diverted to what appeared to be carefully chosen but largely irrelevant results. But by Tuesday, the regular method of blocking was back in place.

Some had hoped that with a new slate of top leaders in place this year, the party would reconsider its long-standing position on Tiananmen — that the protest was a counterrevolutionary rebellion and that the crackdown was necessary for stability.

Over the weekend, the government responded furiously to a statement by the U.S. State Department calling for a full accounting of those killed in the crackdown; the estimates range from hundreds to 7,000.

In a response available only via the government-run Xinhua News Agency’s English platform, Ministry of Foreign Affairs spokesman Hong Lei said Saturday that “a clear conclusion has already been made concerning the political turmoil that happened in the late 1980s.” The United States should stop its “rude interference in China’s internal affairs,” Hong added.

On Friday, a group representing parents whose children were killed at Tian­anmen released an open letter criticizing new Chinese President Xi Jinping for maintaining the party’s stance on the crackdown and continuing to persecute Tiananmen survivors.

Ding Zilin, a former Peking University professor who heads a group of Tiananmen mothers, could not be reached Tuesday by cellphone, home phone or e-mail and was reported by other activists to be under house arrest.

Meanwhile, tens of thousands of people gathered in a Hong Kong park for a vigil Tuesday night, the Associated Press reported. The attendees held candles in remembrance of those slain.

Earlier, some Hong Kong reporters trying to interview people in Tiananmen Square said they were detained for almost an hour.

Hu, who has spent many previous June 4 anniversaries either in prison on state subversion charges or under house arrest, said the security seemed even heavier this year.

“They started even earlier,” he said, noting that authorities began following him and restricting his movements as early as May 25. “I don’t think China’s new leadership has resulted in any improvements at all.”