Beijing tightened security around Tiananmen Square on Wednesday, the 25th anniversary of the June 4, 1989, crackdown on student-led protests.
Dozens of riot police and police patrol cars could be seen parked around the square, as well as at intersections several blocks away on the avenue of Eternal Peace.
"This time 25 years ago today you could not enter Tiananmen Square as a journalist. And 25 years on you still can't," said Al Jazeera's Adrian Brown, reporting from Beijing.
"This really is a reflection of the official nervousness for this very sensitive anniversary. They are trying to erase all public memory of what happened here 25 years ago."
Despite the date's sensitivity, operations at the square seemed to resume as they would on a normal day in Beijing.
However, there was a different situation in Hong Kong where people were able to gather in their tens of thousands to commemorate the historical day due to freer legislation provided by the semi-autonomous status of the city.
Al Jazeera's Rob McBride, reporting from Hong Kong, said: "It is an annual event here to keep alive symbolically the flame of democracy that was lit 25 years ago. People here think they have a bond with those students who came out in Tiananmen Square all those years ago."
People detained and missing
In mainland China, activists and commentators said the country's determination to quell any mention of the event was especially strong this year, and authorities had made mass arrests just before the anniversary.
Rights group Amnesty International said 48 people had been detained, placed under house arrest, questioned by police or had gone missing ahead of the anniversary.
In an apparent sign of government nervousness, connections to the internet appeared to have been disrupted leading up to the anniversary, with Google's mail and other services mostly inaccessible.
Police have also warned foreign reporters from visiting "sensitive places" and prevented them from interviewing people on the topic.
The government has never issued a complete and formal account of the crackdown and the number of casualties.
William Nee, China researcher with Amnesty International rights group, told Al Jazeera that the government keeps the facts about the event secret is because otherwise it would have to conduct a thorough investigation, hold the people responsible for the violence accountable and pay contributions to the relatives of the victims.
"Tiananmen mothers have been demanding full investigation and compensation based on the findings to the victims and their families. And third, to have accountability, said Nee.
"The parents have very powerful stories to tell and I think that is the main reason why the government is going to incredible length to censor all information about what happened."
Beijing's official verdict is that the student-led protests aimed to topple the ruling Communist Party and plunge China into chaos.
The anniversary comes as China grapples with challenges such as slowing economic growth, pollution, endemic corruption and violence in the troubled western region of Xinjiang.