Prominent Chinese activist Xu Zhiyong was tried Wednesday for his role in anti-corruption protests but defied the court by refusing to speak, his lawyer said, calling the proceedings a "piece of theatre".

Prosecutors called for Xu, founder of the New Citizens Movement, to be sentenced to the maximum five years in jail for "assembling a crowd to disrupt order in a public place".

He was accused of being involved in demonstrations featuring banners calling for asset disclosure by officials -- seen as a key measure against graft.

Dozens of uniformed and plain-clothes police surrounded the court in Beijing, with some physically pushing reporters away.

Several Xu supporters said at least three protesters nearby had been detained.

Xu's lawyer Zhang Qingfang told AFP he and his client had remained silent inside the court. "We don't want to take part in a piece of theatre, we are not actors, we can't act," he said.

"The court tried to persuade Xu to speak... and spent 10 minutes trying to persuade the lawyers to speak," added Zhang, who says the trial is procedurally flawed and the evidence was not sufficient to prove the crime.

"Xu maintained silence throughout the trial."

The trial was completed in a six-hour hearing, he said, adding no date was set for the verdict.

Xu is one of several New Citizens Movement activists -- including businessmen and a lawyer -- due to stand trial this week in what has been viewed as part of a government crackdown on dissent.

They are all almost certain to be found guilty by China's politically controlled courts.

The trials come despite a much-publicised anti-corruption drive by China's Communist party under Xi Jinping, and overseas rights groups have condemned the proceedings as hypocritical.

Also Wednesday, an overseas investigation said relatives of top Chinese leaders leaders including President Xi Jinping and former premier Wen Jiabao have used offshore tax havens to hide their wealth.

At least 20 supporters from across China gathered near the courthouse, with a group of around 10 unveiling a red banner calling for officials to disclose their assets -- similar to the actions which led to the charges against Xu.

"If you don't expose your assets, it shows you must have a secret," said one of them, Wu Guangzhong. "Declaring assets is the most common way of preventing corruption."

Zhu Jiaqi, from Tianjin, added: "It should be an open trial but it's held in secret. Xu's voice reflects the voice of the basic and ordinary people."

Repeatedly detained

Foreign diplomats were allowed into the court building, but were told they would not be permitted to attend the hearing itself, one European representative told AFP.

BBC footage showed men wearing hoods wordlessly forcing a journalist and cameraman backwards along a pavement, while a CNN reporter said on Twitter that officials broke his camera.

China's foreign ministry spokesman Qin Gang said journalists entered "a restricted area, so the managing staff removed them from the premises".

Xu's trial was "a reflection of China's rule of law and judicial independence", he said, adding: "China's judicial bodies are trying him in accordance with the law."

Between 20 and 40 people involved with the loose-knit New Citizens Movement have been detained since last year, according to members, while at least three have previously been put on trial.

The arrests have been seen as part of a wider campaign to enforce ideological unity since Xi became the head of the Communist party in late 2012.

Xu trained as a lawyer at one of China's top universities and became widely known in 2003 because of a campaign against a form of extra-legal detention that resulted in the law being changed.

He was at the core of an emerging group of "rights defence" lawyers, who sought to use legal arguments and court cases to push for political reform.

But Chinese authorities, who do not permit independent and organised forms of dissent, arrested him in 2009 on tax evasion charges, which were dropped months later after a public outcry.

In recent years Xu -- whose wife gave birth to a daughter this month -- lived under regular surveillance. He has been repeatedly detained and held under house arrest by state security agents, who even attended his wedding in 2011.

Standing outside the courthouse, Ma Zhurong from the northern province of Shaanxi praised Xu for helping people like her pursue individual justice.

"Xu Zhiyong is just someone who helped powerless people using the law," she said. "(He) wanted to speak up for those who suffered."