A prominent Chinese legal activist defiantly denounced a court as "absurd" on Friday as it upheld his jail sentence for supporting anti-corruption protests, his lawyer said.

Xu Zhiyong, 40, was sentenced to four years in prison in January for backing demonstrations in which a handful of activists held up banners calling for government officials to disclose their assets, as Beijing cracks down on a burgeoning rights movement.

Beijing's high court rejected his appeal and upheld its original verdict, his lawyer Zhang Qingfang told AFP, but his client remained resolute.

"This absurd judgment cannot halt the tide of human progress," he said Xu told the court. "The communist dictatorship is bound to disperse like haze, and the light of freedom and justice will illuminate the East."

The legal scholar is a founder and central figure in the New Citizens Movement, a loose-knit network which campaigns on corruption, access to education and other issues.

China has put Xu and 10 other members of the movement on trial this year on charges of "gathering a crowd to disturb public order" over the protests in 2013.

"This is absolutely an illegal and ridiculous decision," Zhang said of the appeal ruling.

London-based human rights group Amnesty International said it was a "mockery of justice".

"Xu Zhiyong is a prisoner of conscience and he should be released immediately and unconditionally," Amnesty researcher William Nee said in a statement.

"The authorities must end this merciless persecution of all those associated with the New Citizens Movement."

- 'deepening pattern of arrests' -

US State Department spokeswoman Jen Psaki expressed disappointment at the ruling, branding it "retribution for his public campaign to expose official corruption", and called for his immediate release.

"We remain deeply concerned that the prosecution of Xu and others is part of a deepening pattern of arrests and detentions of public interest lawyers, Internet activists, labor activists, journalists, religious leaders and others who peacefully challenge official Chinese policies and actions," she added.

The US and the European Union condemned the original verdict against Xu and a European diplomat said representatives from at least 10 countries attempted to observe the appeal hearing, but were denied access by police.

Police "grabbed and shoved" at least one diplomat outside the courthouse, said the envoy, who declined to be named because of the sensitivity of the case.

Asked about the accusation, Beijing's foreign ministry spokesman Hong Lei told a regular briefing that China "safeguards the legitimate rights of diplomats" but also "requests that diplomats respect China's laws".

"The ruling was made by the judicial authorities in accordance with the law," he said of the court's decision, adding that "China is a country governed by the rule of law".

China's ruling Communist Party is in the midst of a highly-publicised anti-corruption campaign, which President Xi Jinping has pledged will target both high-ranking "tigers" and low-level "flies" in the face of public anger over the issue.

But the party has cracked down harshly on independent activists who have the same goals, viewing independently organised anti-corruption protests as a challenge to its rule.

Official asset disclosure is seen by some as a key reform which would help tackle graft, amid reports that some government officials have amassed vast wealth.

- pushing for political change -

Three other activists associated with the New Citizens Movement went on trial in China's tightly controlled courts this week, and face maximum five-year jail sentences for their role in the anti-corruption protests.

New Citizens Movement activists have said the wave of arrests -- which began last year -- has heavily curtailed their group.

At its peak it had an estimated several hundred participants, who organised dinner discussion meetings across China.

Xu studied law at the elite Peking University and in the last decade became one of an emerging group of "rights defence" lawyers, pushing for political change through court cases.

China has made significant reforms to its legal system since the 1970s and says it guarantees its citizens' legal rights, but it continues to use the courts to control protesters and intellectuals speaking out against government policy.

Xu came to nationwide prominence in 2003, campaigning against a form of extra-legal detention allowing police to detain people arbitrarily if they travelled away from their rural hometowns. The law was ultimately changed.

He went on to provide legal aid to several defendants deemed sensitive by the ruling party, including blind human rights lawyer Chen Guangcheng -- who later made a spectacular escape from house arrest -- and families who sued after their children were poisoned by toxic milk powder in 2008.

Associates of Xu released a message from him on Friday, in which he stated: "Let us in the depths of our hearts, in our daily lives, on the Internet, and on every part of this vast nation, firmly and loudly declare the identity that rightfully belongs to us: I am a citizen, we are citizens."