Police arrested thirteen people who were trying to pay a birthday visit Saturday to blind Chinese lawyer-activist Chen Guangcheng amid a campaign to free him from house arrest, supporters said.

Altogether more than 40 people were blocked by police as they tried to reach Chen, would-be visitors said from a bus near his home.

The self-taught lawyer and rights campaigner, who has been blind since childhood, turned 40 on Saturday and has been under house arrest since ending four years in prison in 2010.

He is famous for revealing forced sterilisations and late-term abortions affecting thousands of women in his home province of Shandong as part of measures to enforce the country's population control policy.

"About a mile after we left the motorway, police diverted us to the bus line no. 12 car park" in the city of Linyi in Shandong, supporter Zhang Fuying told AFP on Saturday by telephone from the bus, which set off from Beijing at 6am.

"We have not come to ask Chen to help us. We are here for human rights in China, for which Chen has done so much. He has the right to eat a piece of cake with ordinary people."

Zhang said that of about 44 people who made the 600-kilometre (350-mile) bus ride to wish Chen a happy birthday, "thirteen have now been taken away by police, and there are 31 people on the bus".

More than 30 police vehicles were parked near the bus, he added, saying that the group were followed when they went to the bathroom and had been unable to buy food since the morning.

Local authorities said when contacted by AFP that they did not know about the events, which follow a series of attempts by journalists and supporters to visit Chen at home as part of a campaign for his freedom.

"Chen has always been interested in the poor and vulnerable -- now we must take an interest in him," said fellow supporter Mao Hengfeng, also speaking from the bus.

Another passenger, Deng Zhibin, made a play on the similarity between the Chinese words "hexie" -- harmony -- and "hexue" -- to drink blood, telling AFP: "This harmonious society is actually a bloodthirsty society."

Hong Kong-based Chinese Human Rights Defenders said that 150 people had gathered on Wednesday at People's Square in Shanghai, then in a restaurant, to celebrate Chen's birthday.

Activists organised through the Internet have been flocking to Chen's village in a bid to win his release, but campaigners say thugs have beaten up many of those who were able to come close.

Foreign journalists who have tried to visit Chen at his home have been roughed up or harassed and barred from gaining access to the village, while earlier this year Chen's wife said her husband had been beaten and threatened.

After his release from prison, Chen made a daring video, smuggled to the US-based group ChinaAid, in which he said police threatened to beat him or throw him back in jail if he spoke out.

China's state-run Global Times newspaper has labelled the campaign surrounding Chen an overreaction to a local issue.

Last week Hillary Clinton raised Chen's case ahead of Pacific Rim talks, saying the US was "alarmed" by Chen's continued house arrest.

"We continue to call on China to embrace a different path," she said.

China's foreign ministry dismissed the remarks as "foreign interference into China's internal affairs and judicial sovereignty".