HONG KONG (Reuters) - Police in Shanghai questioned three dissidents on the ninth anniversary of the crackdown on the pro-democracy movement in Tiananmen Square, a Hong Kong-based human rights group said on Friday. The Information Centre of Human Rights and Democratic Movement in China named the three as Cai Guahua, Yao Zhenxian and Zhou Jianhe. The group said that Cai and Yao were released after five hours of questioning on Thursday, but added that they believed Zhou was still being held in detention. The group said the police moves were made to prevent the activists from staging any commemoration of the Tiananmen Square massacre. But the group said that despite tight police control, Yao still managed to join six other activists for a discussion meeting in a park to mark the anniversary of the Tiananmen killings. The seven dissidents called on the Chinese Communist leadership to re-evaluate the verdict on the student-led Tiananmen Square movement and release all jailed pro-democracy activists. On June 4, 1989 hundreds, possibly thousands, were killed in an army crackdown of the pro-democracy protests centred on Beijing‘s Tiananmen Square. Beijing has steadfastly defended the decision to send in the army, saying it suppressed a „counter-revolutionary rebellion“ aimed at toppling the Communist government. U.S. President Bill Clinton said on Thursday that he intended to stick to plans to participate in a welcoming ceremony at Tiananmen Square during a trip to China this month, despite protests from many U.S. Congress members and exiled Chinese dissidents. „It would be the diplomatic equivalent of dancing on the graves of the courageous and innocent victims of Tiananmen Square“, said New Jersey Republican Rep. Chris Smith. The Information Centre also said police in southern China stopped Xu Wenli, a leading figure in the 1978-79 Democracy Wall movement, from visiting Shenzhen late last month. Xu, who spent 12 years in prison for his outspoken views on democracy, was detained for three days. He returned to Beijing on Thursday, it said.