Hundreds of international academics called Wednesday for the release of scholars and lawyers detained in China ahead of the 25th anniversary of the Tiananmen Square crackdown.

China strictly limits public discussion of the June 4, 1989 suppression of political reform protests, when soldiers killed hundreds of civilians, by some estimates more than 1000.

Authorities have detained dozens -- including lawyers, journalists, artists and relatives of those killed in the crackdown -- ahead of this year's anniversary, according to rights groups and associates.

Around 80 academics from countries including the US, France, Britain, Australia and Germany published an open letter to Chinese President Xi Jinping calling for the release of five people detained last month after they attended a private seminar discussing the crackdown.

The five -- detained on charges of "picking quarrels and provoking trouble" -- include prominent human rights lawyer Pu Zhiqiang, as well as academics.

Several of Pu's associates have been taken into custody in recent weeks -- suggesting that prosecutors are preparing a criminal case against the lawyer, who won plaudits for representing victims of labour camps.

"It is obvious that none of the above-mentioned citizens has committed a criminal offence. Their detention is an injustice," said the letter, signed by a host of prominent China scholars and verified as authentic by AFP.

"We therefore ask you respectfully to correct this mistake, and to free unconditionally the citizens who have been wrongfully detained," it added.

In separate letter released online and addressed to "Chinese citizens," some 160 Japanese, Taiwanese and South Korean academics and writers said they were "deeply worried" about those detained.

"They have carried out rational and peaceful intellectual activities to cure the mental scars Chinese society received after the unfortunate incident in 1989," it said.

"We wish that their bodies and lives will not be under threat."

The US, EU and the United Nations human rights chief have also called for those detained in the run-up to the anniversary to be released.