Human rights groups yesterday called for an independent inquiry on the eve of the 15th anniversary of the Tiananmen Square massacre, with all debate on the issue in China quashed, dissidents under house arrest and phone lines cut. Amnesty International led the charge in a strongly worded statement demanding those responsible for the deaths of hundreds, some say more than a thousand, must be held responsible. "Those found responsible should be tried and brought to justice," the London-based group said. Amnesty "calls on the government to release all those who are still held in connection with the Tiananmen crackdown and who never received fair trials," it added. The group has records of more than 50 people it believes remain imprisoned in China for their part in the protests, a number which is thought to be "a fraction of the true figure" and one never released by the authorities, the statement said. In the lead-up to the anniversary, China's secretive state security police have placed known dissidents under house arrest or strict surveillance and are monitoring universities to prevent commemorations taking place. Telephone conversations with leading dissidents were cut off usually within five or 10 seconds. Some previously used phone numbers are no longer in service. "I'm under surveillance 24 hours a day. A car is parked right outside my home. The officers are of course all plain-clothes," said Ding Zilin, who heads the Tiananman Mothers group and was one of the few people able to speak. At universities, academics said security police have set up a special task force to clamp down on students and dissidents "The universities are under strict control," one academic said. Many people are afraid to talk openly about the events 15 years ago. "It is not convenient for me to talk about this," said a young woman on vast Tiananmen Square, where plain clothes and uniformed police mingled with tourists on what appeared to be an otherwise ordinary day. The leadership has shown no sign of changing its position on the crackdown, defending its actions this week as necessary for economic growth and China's emergence on the world stage. "We strongly condemn such ruthless and vicious tactics against the patriotic students who protested in the streets against corruption in 1989," said the letter signed by the likes of top student leaders Wang Dan and Wang Youcai. "We hope the Chinese government will ensure that China is governed by law rather than by a few individuals, and thereby speed its incorporation into the international mainstream." **PHOTO CAPTION*** A Chinese policeman grabs a protester in Beijing's Tiananmen Square on the 15th anniversary of a bloody military crackdown on democracy protesters, Friday June 4, 2004. (AP)