Chinese police are out in force in a bid to stop any commemoration of the crackdown on protesters in Tiananmen Square 20 years ago. Tanks rolled into the square before dawn on June 4, 1989, to crush weeks of student and worker pro-democracy protests. Hundreds, possibly thousands, of people died in the Chinese government crackdown, yet open discussion of the event remains taboo in China. The number of deaths is disputed but the Chinese government figure of 241 is widely believed to be too low. The tanks of the People's Liberation Army took two days to disperse demonstrators and kill hundreds of unarmed people in streets near to Tiananmen Square. Amnesty International is renewing calls for a public inquiry and is urging its members to hold candle-lit vigils throughout the week. The killings severely bruised relations between the West and Beijing, and there have been echoes of those tensions. US Secretary of State Hillary Clinton called on China to release all those still imprisoned in connection with the protests, to stop harassing those who took part and to begin a dialogue with the victims' families. "A China that has made enormous progress economically and is emerging to take its rightful place in global leadership should examine openly the darker events of its past and provide a public accounting of those killed, detained or missing, both to learn and to heal," Mrs Clinton said. In a sign of Beijing's mix of confidence and caution, Tiananmen Square is open to visitors, but with hundreds of police and guards present - on the tenth anniversary it was closed to the public. But there have so far been no gestures of protest and many Chinese visitors to the square appear to be oblivious to the sensitive date. However, a photojournalist was stopped from taking pictures and told to erase those he had taken. Authorities have also blocked access to the popular internet site Twitter, online photo sharing service Flickr and email provider Hotmail. Foreign news programmes about the anniversary have been cut. "The leaders would rather just avoid this topic," said Zhang Boshu, a philosopher in Beijing who has urged a public reckoning with the killings. "They know that the 1989 crackdown, shooting their own citizens, was a terrible blow to their legitimacy." Mothers of some of the dead from 1989 have also been prevented from leaving their homes to commemorate their children. "We express our strongest outrage and vehement resistance to the Chinese government's barbaric conduct in brazenly violating international human rights covenants and stripping grieving kin of their rights," the Tiananmen Mothers, who want a full accounting of June 4, said in a statement. Thousands of people in Hong Kong are expected to attend a candlelight vigil to commemorate the victims, as they do every year, and in Taiwan activists will likewise mark the anniversary. The 1989 protests began in April after the death of the liberal reformer Hu Yaobang. On April 22, 50,000 gathered in the square to commemorate Mr Hu, a former general secretary of the Communist Party, who had urged democratic reform. The number of people attending rose steadily and the demonstration turned from a commemoration into a broader protest against the government. By the end of April more than a million people had gathered in the square at the centre of Beijing.