China is marking the 24th anniversary of the bloody Tiananmen Square crackdown, amid tight security in Beijing and stifling censorship on the web.
Authorities every year work hard to prevent memorials and ban public discussion of the brutal military suppression on June 4, 1989, which ended weeks of pro-democracy demonstrations.
On Friday, police in Tiananmen Square and other prominent areas stood on guard for possible protests. Many activists have already been detained, placed under house arrest, or monitored closely in the lead-up to the sensitive anniversary.
Tiananmen Square, Beijing
Government censors are also working hard to scrub China's social media of any mention of the incident. On the popular, Twitter-like Sina Weibo, searches for all Tiananmen-related terms were blocked. The service even removed a candle icon used by many as a digital vigil.
Attempting to get around the restrictions, many Chinese citizens instead posted pictures of candles, or cynically referred to May 35, rather than June 4 - a search term that is also blocked. Others encouraged people to wear black as a symbol of mourning for the victims of the incident.
In Hong Kong, more than 100,000 people are to attend a candlelight vigil to remember the crackdown. Memorials and protests are held around this time every year in the former British colony, which enjoys a greater degree of civil liberties than on the mainland.
It has been 24 years since Chinese troops, backed by tanks, moved in to crush a student led demonstration centered in Tiananmen Square. The crackdown triggered worldwide condemnation, with estimates of those killed ranging from several hundred to several thousand people.
China still considers the incident a "counter-revolutionary rebellion" and has never admitted any wrongdoing in its handling of the uprising. It has never disclosed an official death toll or other key details on the crackdown, which is not discussed in state media.
Late last month, the U.S. State Department again called on Beijing to "end harassment of those who participated in the protests and fully account for those killed, detained, or missing."
China's foreign ministry responded by saying the statement represented an interference into its internal affairs and warned the issue could "sabotage China-U.S. relations."
Human rights are expected to be discussed later this week during a summit between U.S. President Barack Obama and Chinese President Xi Jinping, who is visiting the western state of California.