BEIJING -- Authorities in Beijing blanketed the city centre with heavy security Wednesday on the 25th anniversary of the bloody military suppression of pro-democracy protests centred on Tiananmen Square.

Police and paramilitary officers patrolled the vast plaza and surrounding streets, stopping vehicles seeking to enter the area and demanding identification from passers-by. Reporters were told to leave the area following the usual dawn flag-raising ceremony.

Dozens of activists, dissidents and other critics have already been detained by police, held under house arrest or sent out of the city.

China allows no public discussion of the events of June 3-4, 1989, when soldiers backed by tanks and armoured personnel carriers fought their way into the heart of the city, killing hundreds of unarmed protesters and onlookers. The government has never issued a complete, formal accounting of the crackdown and the number of casualties.

However, authorities were allowing relatives of some of those killed in the crackdown to visit their graves under police escort, according to Zhang Xianling, a member of a group that campaigns for the crackdown's victims.

"Even though 25 years is a very long time, as a relative, as a mother, it feels like this happened just yesterday," said Zhang, whose son Wang Nan death was 19 years-old when he was killed in the suppression.

"The wound is still very deep. And though we might now shed fewer tears than in the past, our conviction is even stronger," Zhang said. "We must keep struggling until the end. We must pursue justice for our loved ones."

Beijing's official verdict is that the student-led protests aimed to topple the ruling Communist Party and plunge China into chaos. Protest leaders said they were seeking greater democracy and freedom, along with an end to corruption and favouritism within the party.