BEIJING (NYTIMES) - Friends of an artist who fled Beijing after documenting mass expulsions of migrant workers from the Chinese capital say he has gone missing after being detained by the police.

The painter, Hua Yong, posted dozens of short videos in recent weeks recording how the authorities had forced tens of thousands of people to leave Beijing and demolished vast swaths of neighbourhoods.

The police came for him after he filmed residents protesting by blocking a roadway in the Daxing district of Beijing, and he fled the city.

On Friday, he posted a series of short videos that he said were filmed in an apartment in the city of Tianjin, a large metropolis near Beijing. In some of the videos, someone can be heard pounding on the apartment's door and telling him to come out.

Hua, 48, who had shaved his beard and shorn his dreadlocks, said his arrest was imminent. As he waited, he recorded himself singing Happy Birthday to his daughter, who turns three this month.

"Everything I do is so your generation won't have to go through what I and your grandfathers' generation experienced," he said in the video to his daughter.

"I want to make our country better. To be just, fair, free, democratic and have freedom of speech."

Ji Feng, a friend, said Hua's arrested had been "confirmed".

"But we haven't been able to figure out anything else. We and his family are trying to make contact with him," he said.

Ji said friends and relatives of Hua did not know where he was being held or what he might be charged with.

Reached by phone, the police in the Daxing district declined to answer questions about Hua.

Hua's work documenting the evictions of migrant workers touched a nerve with the authorities, said Patrick Poon, a Hong Kong-based researcher for Amnesty International.

"His videos became important evidence about the human rights violations during the evictions," Poon said. "His detention makes him become like a symbol about how grassroots people are treated by the Chinese government."

Hua has been incarcerated over speech issues in China before. In 2012, he was sentenced to 15 months in a labour camp after a performance in Tiananmen Square in which he punched himself in the face, then used his blood to write the date of the 1989 massacre of pro-democracy protesters.

In the videos recorded on Friday, Hua repeated his desire to remain in China rather than leave for some place where he would be able to speak more freely about political issues.

"The People's Republic of China Constitution provides for freedom of speech, freedom of the press," he said.

"All I did was take and post a few videos online. There's nothing wrong with this. So I will stay in China. Even if I die, I die in my country."