TAIPEI (REUTERS) - Taiwan's president offered on Sunday (June 4) to help China transition to democracy, on the anniversary of China's bloody crackdown on student-led pro-democracy protests in and around Beijing's Tiananmen Square.

Ms Tsai Ing-wen said that the biggest gap between Taiwan and China is democracy and freedom, needling Beijing at a time when relations between China and the self-ruled island are at a low point.

"For democracy: some are early, others are late, but we all get there in the end," Ms Tsai said, writing in Chinese on her Facebook page and tweeting some of her comments in English on Twitter on the 28th anniversary of the June 4, 1989, crackdown.

"Borrowing on Taiwan's experience, I believe that China can shorten the pain of democratic reform," Ms Tsai said.

Beijing distrusts Ms Tsai and her ruling Democratic Progressive Party because it traditionally advocates independence for Taiwan. Beijing says the island is part of China and has never renounced the use of force to bring it under its control.

After nearly 40 years of martial law imposed by the Nationalists on Taiwan, the island in the late 1980s began its own transition to democracy, holding direct presidential elections since 1996.

"When democracy is in front of you, no country can ever go back," Ms Tsai wrote.

China's Taiwan Affairs Office did not immediately respond to a request for comment on Ms Tsai's remarks. The Tiananmen crackdown is a taboo subject in China.

However, tens of thousands of people were expected to gather later in the day in Hong Kong to mark the anniversary, the only place in China where such large-scale public commemorations happen.

Beijing sent tanks to quell the 1989 protests, and has never released a death toll. Estimates from human rights groups and witnesses range from several hundred to several thousand.

On Friday (June 2), Foreign Ministry spokesman Hua Chunying said that China had long ago reached a conclusion about the events of that period.

"I hope you can pay more attention to the positive changes happening in all levels of Chinese society," she said, without elaborating.

In Beijing, security was tight as usual at Tiananmen Square, with long lines at bag and identity checks. The square itself was peaceful, thronged with tourists taking photos.

One elderly resident of a nearby neighbourhood, out for stroll at the edge of the square, said he remembered the events of 28 years ago clearly.

"The soldiers were just babies, 18, 19 years old. They didn't know what they were doing," he told Reuters, asking to be identified only by his family name, Sun.

While some search terms on China's popular Twitter-like microblog Weibo appeared to be blocked on Sunday, some users were able to post cryptic messages.

"Never forget," wrote one, above a picture of mahjong tiles with the numbers 6 and 4 on them, for the month and day of the anniversary.