HONG KONG — A pro-democracy activist in Hong Kong said on Friday that Chinese agents drove staples into his legs after he asked the soccer star Lionel Messi to send a message of support for a jailed Chinese dissident.

The bizarre episode has heightened concerns about the erosion of the rule of law in Hong Kong as the mainland authorities try to exert greater control over the semiautonomous Chinese city’s freewheeling politics.

At a news conference in Hong Kong, the activist, Howard Lam, a member of the Democratic Party, displayed metal staples in his thighs that he said were put there by men who abducted and beat him.

In early July, Mr. Lam wrote to F.C. Barcelona, the Spanish soccer club, to ask for a signed photograph of Mr. Messi, the Argentine soccer star. Friends said that the jailed Chinese dissident, Liu Xiaobo, was a fan of the player and the team, and that after Mr. Lam learned that Mr. Liu had late-stage liver cancer, he sought to send the photo to him. But Mr. Lam did not receive the picture until after Mr. Liu, a Nobel laureate, died on July 13.