Five men who were jailed for supporting the 1989 democracy movement have called on the Chinese government for economic redress, saying they are struggling to survive because of their punishment.

The bold move comes days ahead of the 20th anniversary of the bloody crackdown on the protests, which left hundreds dead in Beijing. The movement began with student demonstrations in Tiananmen Square and spread rapidly across the country.

Harassment of dissidents has increased in the run-up to Thursday's anniversary, with Ai Weiwei, one of the best-known critics of the government, telling the Observer that authorities have shut his blog and placed him under surveillance.

In an open letter to Chinese leaders, released via US-based group Human Rights in China, the five former prisoners from Zhejiang province claim that they are suffering financially because they are still labelled as "June Fourth thugs".

"Since our imprisonment after the 4 June 1989 crackdown, we not only lost our jobs, we were also stripped of the cumulative benefits of our past labour and lost our pension rights," wrote Wu Gaoxing, Chen Longde, Wang Donghai, Mao Guoliang and Ye Wenxiang.

"Some are now past retirement age, yet have no source of income to cover living expenses and no medical insurance; others ... have no choice but to drift from place to place doing temporary manual labour to support their families, while living apart from their wives. If we get sick, we can only wait to die, and all this just because 20 years ago we were sentenced for political reasons."

They ask that former prisoners should receive pensions based on their work prior to the crackdown or state support, while those of working age should be able to return to their old jobs or be given compensation.

Their intervention comes as authorities clamp down on dissidents ahead of the sensitive anniversary.

Bao Tong, the most senior official jailed over the movement and now an outspoken critic of the government, was asked to leave Beijing by security officials last week.

Yesterday Ai, the artist-turned-activist who designed the Bird's Nest Olympic stadium, told the Observer that Sina, China's biggest news portal, closed his blog without explanation on Friday after he refused to self-censor his posts ahead of 4 June.

It came shortly after Ai posted details of altercations with state security officers who he said have started to follow him and intimidate those around him. Four plain-clothes officers interrogated his 76-year-old mother. Another associate was woken at 4am and questioned for hours. Ai said it was absurd because he was in the United States in 1989 and played no part in the Tiananmen protests.

The influential artist's campaign to uncover why so many schools collapsed in the Sichuan earthquake generated wide support, and he has been among the highest-profile online critics of the one-party system. He added: "Freedom of expression is absolutely the most important thing for society. We don't have it in China. Internet and blogging were the only possibility for us to express ourselves. Now it is more restricted than ever."