Chinese President Xi Jinping has chosen "repression over reform" as clampdowns precede the 25th anniversary of the Tiananmen Square crackdown, the head of Amnesty International said Tuesday.

China still forbids public discussion of the events on June 3-4, 1989, when the military brutally suppressed pro-democracy protesters in central Beijing.

Chinese police earlier this month detained prominent rights lawyer, Tang Jingling, the latest of around 20 activists held on criminal charges before the anniversary.

"When President Xi came to power, we heard that there was going to be greater openness," Amnesty International secretary general Salil Shetty told AFP in an interview.

"I would have thought that the 25th anniversary of the Tiananmen Square crackdown would have been a great opportunity for him to convert that sort of claim into some practical reality," Shetty said.

"Unfortunately what we've seen is that he's chosen repression over reform," he said, adding that China should conduct an open and independent investigation into the events of 1989.

China generally rounds up dissidents before dates it considers sensitive, but rights groups say this year's detentions have been unusually widespread.

Police have criminally detained some 20 prominent liberal academics, lawyers and activists in the past month, according to the US-based group Human Rights in China.

"They've used this as an opportunity to settle scores, to effectively establish where the new red lines are going to be," Shetty said.

"They just get away with it, so they keep doing it," said the Amnesty chief, who is visiting Hong Kong to pay respects to the victims of Tiananmen.

Those detained this month include Pu Zhiqiang, one of China's most celebrated human rights lawyers, who was held along with four others who attended a private seminar to discuss the Tiananmen protests.

"People who are essentially trying to remember the victims: there's a crackdown on those people," Shetty said.

Beijing has never provided an official final toll for the military crackdown, which was condemned worldwide. Independent observers tallied more than 1,000 dead in Beijing.

The Chinese Communist Party branded the Tiananmen protests a "counter-revolutionary rebellion", but pro-democracy advocates in Hong Kong have consistently commemorated the incident.

The world's first museum dedicated to those who sacrificed their lives for democracy opened in April in the semi-autonomous southern Chinese city.

Tens of thousands of residents have also gathered for almost a quarter of a century to mark the anniversary in the city's Victoria Park.

"It's absolutely crucial that the people of Hong Kong continue to raise their voice to commemorate the victims...there's no short cut," Shetty said.