BEIJING (AFP) - Chinese police have charged seven activists who held a ceremony to mark the 25th anniversary of the Tiananmen Square crackdown on pro-democracy protesters, rights groups said on Friday.

The activists were formally accused on Thursday of "picking quarrels and provoking trouble" in the central province of Henan, the US-based group Chinese Human Rights Defenders said in a statement citing relatives.

Acquittals in Chinese courts are extremely rare and the offence carries a maximum term of five years in prison.

Chinese authorities strictly limit public discussion of the June 4, 1989 crackdown when soldiers killed hundreds, by some estimates more than 1,000, protesters who had called for political reforms. They detained dozens of activists ahead of this year's anniversary.

Photographs of the Henan memorial ceremony posted online show a handful of people in a field with funeral wreaths laid next to an image of Zhao Ziyang, the liberal Communist party boss who was purged following the crackdown.

Those arrested include former Tiananmen student leaders Chen Wei and her husband Yu Shiwen, and lawyers Ji Laisong and Chang Boyang, who represented a local disability rights organisation, the US-based Human Rights in China group said.

China's ruling Communist party does not tolerate organised dissent, and authorities have increasingly used the vague charge of "picking quarrels and provoking trouble" to target dissidents in recent months.

Beijing police last month announced the charging of one of China's most celebrated human rights lawyers, Pu Zhiqiang, with the same offence after he participated in a private seminar discussing the Tiananmen crackdown.