BEIJING — Heavy security blanketed central Beijing on the 25th anniversary of the bloody suppression of the Tiananmen Square pro-democracy protests today, pre-empting any attempts to publicly commemorate one of the darkest chapters in recent Chinese history.

Scores of police and paramilitary troops patrolled the vast plaza in the city’s heart and surrounding streets, stopping vehicles and demanding identification from passers-by.

Accompanied or otherwise monitored by police, some relatives of the crackdown’s victims paid respects at cemeteries or at home, expressing frustration at being prevented from organizing public memorials.

China allows no public discussion of the events of June 3-4, 1989, when soldiers backed by tanks and armored personnel carriers fought their way into the heart of Beijing, killing hundreds of unarmed protesters and onlookers.

Yin Min, whose 19-year-old son, Ye Weihang, was killed in the crackdown, said she wept in grief as she hugged his ashes at home in the morning.

“I told my son this morning, ‘Your mother is powerless and helpless, after more than 20 years I don’t even have the chance to appeal for support,”’ Yin said in a phone interview from her home in Beijing, crying as she spoke.

“I looked at his ashes, I looked at his old things, and I cried bitterly,” Yin said. “How has the world become like this? I don’t even have one bit of power. Why must we be controlled so strictly this year?”

The government has largely ignored the relatives’ demands for an admission of wrongdoing and for a complete, formal accounting of the crackdown or the number of casualties.

Near the square in Beijing, reporters were told to leave following the daily crack-of-dawn flag-raising ceremony and there were no signs of demonstrations or public commemorations.