The director of Amnesty International UK said she was "forcefully" shoved by a Chinese embassy official during a ceremony to remember protesters killed in one of China's bloodiest crackdowns.
Survivors of China's Tiananmen Square crackdown joined human rights campaigners to lay flowers outside the Chinese embassy in central London to mark the 25th anniversary of the event.
Speaking afterwards outside the embassy, director of Amnesty International UK Kate Allen said: "One of the embassy officials shoved both of us very forcefully in the back so that we both flew into the journalists and photographers standing behind us [...] I think they've shown their true colours."
Exiled Tibetan spiritual leader the Dalai Lama marked the 25th anniversary of the Tiananmen Square crackdown in Beijing by urging China to embrace democracy and offering prayers for the protest "martyrs".
The Dalai Lama, reviled by Beijing as a separatist, made the rare comments on the June 4, 1989, violence at a prayer meeting two years after he renounced politics.
"I offer my prayers for those who died for freedom, democracy and human rights," the Nobel Peace Prize winner said according to a statement posted on his website.
"While great progress has been made to integrate into the world economy, I believe it is equally important to encourage China to enter the mainstream of global democracy," he added.
China's foreign ministry has accused the US of making "irresponsible remarks", after the White House issued a statement calling on the government to account for those who were killed, were detained or went missing during and after the Tiananmen Square protests.
In a statement to reporters, foreign ministry spokesman Hong Lei asked westerners to "respect China's judicial sovereignty", and expressed their "firm opposition" to those who put down China's authority. He added that people were "exhausted" listening to "old news".
Mr Hong said: "What China has achieved in human rights is practical and reasonable." Responding to journalists, he said "it means that our policies in regards to human rights are correct".
The White House has paid tribute to the pro-democracy protesters who stood up against the Chinese government in Tiananmen Square 25 years ago, and called on Chinese authorities to account for those who were killed, were detained or went missing during and after the protests.
Press secretary Jay Carney said the US "will always speak out in support of the basic freedoms the protesters at Tiananmen Square sought".
Mr Carney praised China for "extraordinary social and economic progress" over the last three decades and urged the Chinese government to guarantee "universal rights and fundamental freedoms" to all its citizens.
Japan called on China to guarantee basic values including human rights on the 25th anniversary of the Tiananmen Square crackdown.
"As the Japanese government, we think it is extremely important that the Chinese government guarantees freedom, basic human rights and the rule of law which are universal values in international society," said Japan's top government spokesperson Yoshihide Suga.
Suga added that Japan looks forward to a change in attitude from China. "We have expressed our opinion to the Chinese government on many occasions and we hope that China proceeds in a positive manner," Suga said.
China allows no public discussion of the events of June 3 and 4 1989, when soldiers backed by tanks and armoured 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 or the number of casualties.
Beijing is shrouded in a blanket of heavy security today on the 25th anniversary of the bloody military clampdown on pro-democracy protests, in a preemptive operation designed to prevent any attempts to commemorate the event on Tiananmen Square.
Scores of police and paramilitary troops patrolled the vast plaza in the city's heart and surrounding streets, stopping vehicles and demanding identification from passers-by.
Dozens of activists, dissidents and other critics have already been detained by police, held under house arrest or sent out of the city.
Tiananmen Square has been cleared of people following a fatal incident today that saw a car plough into crowds of people, killing three and injuring many more.
According to an official police microblog, the car veered off the road at the north of the square, crossed the barriers and caught fire, injuring "many" tourists and police.