BEIJING  Wu Yuren, an artist who helped lead an unusually bold public protest last winter over a land dispute, has been languishing in a Beijing jail for almost six weeks after having been beaten by police officers, his wife said on Thursday.

Mr. Wu’s wife, Karen Patterson, a Canadian citizen, said in a telephone interview that the police were accusing her husband of assaulting an officer when he visited the police station on May 31. Ms. Patterson said she learned this only through their lawyer because the police had so far not formally told her that Mr. Wu had been arrested. She decided to publicly discuss the arrest in recent days, she said, because of what she called her frustration with China’s opaque legal system.

“You don’t realize how arcane this system is until you have to deal with it,” Ms. Patterson said. “It’s a nightmare.”

Ms. Patterson said she and friends of Mr. Wu, 39, believe that he had been arrested because of his recent activism, including his leadership of a group of artists from an artists’ district known as 008 in resisting the encroachment of a real estate developer. In February, those artists joined forces with artists from another Beijing neighborhood to march down Chang’an Jie, a wide ceremonial avenue that runs past the Forbidden City and Tiananmen Square. Chinese leaders are especially sensitive to protests in that area, and police officers stopped the protesters after they had walked about 500 yards.