The only anniversary protests were in Hong Kong

Jiang Qisheng, 52, who has already spent a year and a half in prison, was sentenced by a Beijing court for "attempting to subvert state power".

Mr Jiang was arrested in May 1999 after writing an article calling on people to remember those killed during the suppression of the pro-democracy protests in Tiananmen.

He was also accused of trying to overthrow the state by distributing an essay written by another activist which called for political reform.

The BBC Beijing correspondent says the four-year sentence is comparatively light by Chinese standards.

A human rights group says the timing of the sentencing is intentional, with China hoping the West will not notice during the Christmas and New Year holidays.

No visits

Mr Jiang, who has previously been jailed for his role in the Tiananmen protests, said the court's punishment was proof China did not respect freedom of expression.

The 1989 Tiananmen Square demonstration

His wife, Zhang Hong, said she had not been allowed to visit or phone her husband since his detention.

She has accused the court of breaking Chinese law by holding her husband for so long without sentencing him.

When his trial began in November 1999, his attorney expected a verdict within 30 to 45 days.

The charge, "attempting to subvert state power", is a catch-all used to prosecute dissidents of all stripes.

Mr Jiang was a leader of the student protests at Tiananmen Square and served a year and a half in prison for his part in the demonstrations.

After weeks of protest in the central Beijing square, the military moved in to crush dissent in June 1989.

Hundreds of students and other protesters were killed in the crackdown, which put an effective halt to mass pro-democracy demonstrations.