(Recasts with advertisement saluting Tiananmen mothers)

By Lindsay Beck

BEIJING, June 4 (Reuters) - An advertisement saluting mothers of students and workers killed in the 1989 Tiananmen Square crackdown appeared in a newspaper in southwest China on Monday, two witnesses said, in a rare public criticism of the massacre.

The advertisement, in the lower right corner of page 14 of the Chengdu Evening News, read: "Paying tribute to the strong mothers of June 4 victims", two local residents who saw it told Reuters.

Police are investigating how the advertisement got into the newspaper, one local resident who requested anonymity said.

A newspaper editor and a local Communist Party official, reached by telephone, declined immediate comment.

In Hong Kong, some 55,000 people attended a candlelit vigil to remember the dead, according to organisers' estimates.

On June 4, 1989, People's Liberation Army troops and tanks crushed student-led pro-democracy demonstrations centred on Beijing's Tiananmen Square, killing hundreds, possibly thousands of workers and students.

Tiananmen Square was quiet on Monday, China's media were silent about the bloody crackdown that took place in and around the square 18 years ago to the day, but rights groups said it would not be forgotten.

"I want to tell those who claim that Tiananmen 'belongs to another era' that, behind the high, barbed-wire-ringed walls of the Chinese prisons, Tiananmen prisoners are still suffering," the Chinese Human Rights Defenders said in an e-mailed statement, quoting an unnamed former prisoner of conscience.

The subject is still taboo and the government has rejected public calls to overturn the verdict that the protests leading to the crackdown were "counter-revolutionary", or subversive.

The Communist Party's then chief, Zhao Ziyang, was sacked for opposing the crackdown and died in 2005 after spending more than 15 years under house arrest.

"TRAGEDY FOR MANKIND"

Bao Tong, a top aide of Zhao, told Reuters that China could never forget the events of 1989.

"To forget June 4 is a national humiliation," he said in a rare interview at his Beijing apartment. "June 4 was a tragedy not just for China but for the whole of mankind. We will still be talking about June 4 in 1,000 years' time."

Tiananmen Square itself was full of the usual tour groups, both Chinese and foreign, and locals flying kites, as well as plainclothes security and police vans.

At its north end, in front of the giant portrait of Mao Zedong, police surrounded a woman and rifled through her bag before bundling her into the back of a van, a sign of the tight rein China keeps on petitioners and protesters.

Despite activists' efforts to keep memories of the events alive, the virtual silence on that period within China means many people know little about the Tiananmen movement.

One vendor surnamed Zhang, hawking copies of Mao's Little Red Book, at first said he had no idea of the sensitive anniversary, only recalling it after some prompting.

"I know what you mean," he said finally. "But I come here every day and there's nothing like that now."

"I have no idea what you're talking about," said a woman, 26, visiting from the southeastern port city of Xiamen with her husband and toddler.

Ding Zilin, who leads the pressure group Tiananmen Mothers, was allowed for the first time to lay flowers at the spot where her son was shot and hold a brief memorial ceremony, said the Hong Kong-based Information Centre for Human Rights and Democracy.

In Hong Kong, tens of thousands of people turned out for the annual candlelit vigil.

"It is a matter of principle. What is wrong is wrong. As long as China doesn't admit any wrongdoing, I will keep coming to the vigil," said 46-year-old graphic designer William Tam.

Key figures have been silenced at home or forced into exile abroad since 1989, but voices for reform have mutated into a crusade involving a new generation of civil rights campaigners. (Additional reporting by Benjamin Kang Lim and James Pomfret, David De Sola and John Ruwitch in Hong Kong)

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