The detention of Guo Jian is the latest in a string of detentions of lawyers, artists, scholars and journalists ahead of the 25th anniversary of the military clampdown on the pro-democracy student movement centred around Beijing's Tiananmen Square.
Discussions of the protest and its military suppression have been a taboo in China, and the authorities always tighten security ahead of the anniversary each year.
But this year's suppression has been harsher than in previous years, and Beijing has shown far less tolerance with the activists, who had got no more than warnings in the past for their restrained commemoration.
The Europe Union expressed its concern in a recent statement.
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"While acknowledging China's advancement of the economic and social wellbeing of its people in the past 25 years, the EU also hopes to see greater space open up for discussion and debate about China's recent history and for lawyers, human rights defenders and journalists to carry out their work without harassment or interference," the statement said.
Fairfax Media said Guo, a former soldier and protester, was taken from his home in suburban Beijing on Sunday night. Australia's Department of Foreign Affairs and Trade issued a statement saying it is seeking information on Guo's reported detention and is ready to provide him with consular assistance.