Chinese overseas are reporting they are being blocked from posting on a popular Chinese social networking site.
Three people said on Twitter that their attempts to post on Weibo were greeted with an error message saying that users with an overseas IP address could not make posts on the China-based service.
China restricts information before and during the 30th anniversary of a deadly military crackdown on pro-democracy protesters in Beijing's Tiananmen Square.
There is virtually no mention in mainland China of the crackdown that took place the night of June 3-4, 1989, and is believed to have killed hundreds and possibly thousands of people.
Weibo did not respond immediately to a request for comment.
Pro-democracy activists in Hong Kong are preparing for a candlelight vigil commemorating the Tiananmen protests and remembering the victims of the crackdown.
The semi-autonomous territory is the only region under Beijing's jurisdiction which has a significant public commemoration of the 1989 crackdown. Hong Kong has a degree of freedom not seen on the mainland as a legacy of British rule. The event beginning at 8 p.m. Tuesday is expected to attract tens of thousands of participants.