In this photo taken July 1, 2009, artist Ai Weiwei signs an autograph at an event in Beijing. Ai has been collecting names of all the schoolchildren who died in the May 2008 earthquake in Sichuan province. ((Greg Baker/Associated Press))

Chinese artist Ai Weiwei is recovering from surgery in Germany after suffering a brain injury he says was inflicted by police in China.

The artist, who has posted pictures of himself on Twitter, said a surgeon in Munich told him he needed immediate surgery because of bleeding in the intercranial cavity between his brain and skull.

The 52-year-old artist had the operation Monday.

The avant-garde Beijing-based artist is in Germany ahead of his October exhibit there. He said he'd suffered dizziness ever since being punched in the head and detained in a hotel in Sichuan province in August.

Twenty uniformed and plainclothes police converged on the hotel where he was staying and threatened to kill him, he said.

5,000 schoolchildren died in Sichuan quake

Ai has been a public critic of the Chinese government's version of events over the Sichuan earthquake in May 2008 in which 87,000 people died or were never accounted for. At least 5,000 of them were schoolchildren.

Many blame poorly constructed schools that collapsed during the catastrophe.

Ai has been collecting and publicizing the names of all the students who died in the quake. The artist said he was back in Chengdu, Sichuan, to testify as a witness in a case involving fellow quake researcher Tan Zuoren, who is charged with subversion and revealing state secrets.

Ai was also there to continue his research on the names of children who died in the quake.

"The authorities were very nervous and tried to prevent us from doing our research," Ai told German magazine Spiegel last month about his experiences in Sichuan.

Ai's exhibition "So Sorry," at Munich's Haus der Kunst, will open Oct. 12 as planned, according to the museum, which sent out a statement about his injury.

His work has been exhibited around the world, including the Venice Biennale.

Ai was the original designer of the massive Olympic Beijing National Stadium, also known as the Bird's Nest, but then distanced himself from it and the Games.