Image copyright Freedom for Liu Xiaobo Action Group Image caption The activists held a seaside memorial for the late dissident last week

At least four men have been detained by Chinese police after publicly commemorating the death of Nobel peace laureate Liu Xiaobo, an activist says.

The seaside memorial, which took place on 19 July, was part of a widely shared international campaign on social media mourning the dissident's death.

Wei Xiaobing, He Lin, Liu Guangxiao and Li Shujia are being held at a Guangdong detention centre, activist Hu Jia says.

Another activist who took part, Wang Xin, was missing, he added.

"The detentions are illegal," Mr Hu told BBC News. "There is no Chinese law that sanctions the punishment of a person who memorialises a death, no matter who that person was."

The four men had been informed by police that they had allegedly disrupted public order, said Mr Hu, who is a long-time Beijing-based activist and a member of the Freedom for Liu Xiaobo Action Group.

Last week, the group organised a global social media campaign asking supporters of Liu Xiaobo to post photos of an empty chair next to the sea with the hashtag #withliuxiaobo.

Mr Liu, an academic who became a pro-democracy campaigner, was represented at his Nobel Peace Prize ceremony in 2010 by an empty chair because he was in prison in China.

Image copyright AFP Image caption Liu Xiaobo helped draft a democracy manifesto for China and was subsequently jailed for "subversion"

The campaign started on 19 July, seven days after Mr Liu died from liver cancer in the northern Chinese city of Shenyang.

The seventh day is an important part of traditional Chinese funeral rites. That is the day when the deceased is believed to return to his or her home to say goodbye to their loved ones.

The Chinese Nobel laureate was cremated, and his ashes were scattered into the sea.

His supporters say the authorities wanted to avoid providing a burial ground where he could be commemorated.

Mr Hu said about 10 activists participated in last week's memorial, which took place at a beach in Guangdong province and was broadcast live by a news channel in Hong Kong.

In photos shared on social media, the activists held up three fingers to symbolise resistance, freedom and hope.

Mr Hu said some of the other activists who had not been detained were currently on the run.

He said the first of the group to be detained was Wei Xiaobing, who was apprehended in the early hours of 22 July. The others were detained shortly afterwards.

Liu Xiaobo's death has been marked in nations around the world but in China authorities have clamped down on commemorations, both in public and on social media.