By James Pomfret and Terril Yue Jones

HONG KONG/BEIJING (Reuters) - Tens of thousands of people held a candlelit vigil in a rain-soaked Hong Kong park on Tuesday to urge China to respect human rights on the 24th anniversary of the bloody crackdown on pro-democracy protesters in Beijing's Tiananmen Square.

In China, most mention of the anniversary was censored from the Internet. Security was tight in Beijing, where on June 3 and June 4 1989, China's leaders ordered troops to open fire on demonstrators and sent in tanks to crush a student-led campaign movement, killing hundreds.

In Hong Kong, members of the crowd wore black and held candles under umbrellas. Protesters demanded Beijing overturn its denunciation of the pro-democracy movement as a "counter-revolutionary event".

"Vindicate June 4th!" many shouted. "Never give up!"

Hong Kong, a former British colony that returned to Chinese rule in 1997, is the only place on Chinese soil where large, open commemorations of the Tiananmen massacre take place. The vigil is held up as a symbol of Hong Kong's relative freedoms and civil liberties compared with mainland China.

"I feel very sad. It's been 24 years and nothing has changed. The changes that the students of 1989 demanded have not come yet. Instead, things are getting worse and worse. You can see the officials are still very corrupt," said Doris Poon, a clerk in her late 40s.

Organizers estimated 150,000 people showed up to send a "united message" to China's new president, Xi Jinping, whose government has clamped down on Internet freedoms and detained anti-corruption activists. Police put the figure at 54,000 at the peak of the assembly.

A record 180,000 people attended last year's vigil.

Xi became Communist Party chief in November and president in March at a time of mounting public pressure for long-stalled political reforms, though there has been no sign of that so far.

While China grapples with thousands of protests a year, over everything from pollution to corruption and illegal land grabs, none of these demonstrations has even come close to becoming a national movement that could threaten the party's rule.

"Everybody can see that China today continues to tighten and this suppression of human rights will cause more Hong Kong people to come out," said pro-democracy lawmaker Lee Cheuk-yan.

SECURITY TIGHT IN BEIJING

In Beijing, Tiananmen Square was packed with tourists on a cool, smoggy day. Plain clothes security and police were out in force, checking identity cards of Chinese tourists.

"It's June 4, a day the Communist Party and the Chinese government don't like," said a young woman from the southern city of Shenzhen, who declined to give her name.

"Older people who remember told us about it. We know June 4. It's called the day students shed blood."

The government last week began detaining some dissidents and cutting off telephone and Internet access for others.

"If the government is sensible, next year is the 25th anniversary and they could designate a spot where we could march," said Zhang Xianling, 76.

Zhang is one of a group of "Tiananmen Mothers" who seek justice for children killed in the crackdown and were closely watched by police as they paid respects to victims in Beijing's Wan'an cemetery.

Last week, the group denounced Xi for failing to launch political reforms and accused him of taking china "backwards to Maoist orthodoxy".

"If you want to commemorate it, you should be able to commemorate it," Zhang said. "That would be an enlightened government."

Authorities boosted censorship of social media sites, blocking searches not only for terms like "Tiananmen" but even the words "today" and "tonight", and removing the candle symbol from the list of emoticons on the Twitter-like service Sina Weibo.

A satirical, doctored image of the famous photograph depicting a man confronting a tank - replacing the line of tanks with giant yellow rubber ducks - was circulated online before it was blocked by censors.

Some skirted the restrictions through cryptic allusions.

"The events of that day in that year I will never forget. I will always keep a candle lit in remembrance in my heart," wrote one Weibo user.

(Additional reporting by Venus Wu, Lavinia Mo and Stefanie McIntyre in Hong Kong and Ben Blanchard in Beijing; Editing by Anne Marie Roantree and Andrew Heavens)