Hong Kong police have arrested a pro-democracy activist who said mainland Chinese agents had stapled his legs on suspicion of providing false information.

Howard Lam made headlines last week with his allegations, which rekindled fears about Beijing interfering in Hong Kong despite promising it considerable autonomy since the 1997 handover from Britain.

But police did not believe his story. “The victim’s reports about his activities on that day and the investigation’s results do not match,” they said. “At this time, there’s no evidence that anyone was illegally detained in Hong Kong.”

Lam, 42, had intended to send a signed photo of the Argentinian footballer Lionel Messi to Liu Xia, widow of the Nobel peace prize laureate Liu Xiaobo, and posted his plans on Facebook. He said he received a call early last week from an acquaintance on the mainland warning him not to send the image.

Lam said on Thursday, unknown Mandarin-speaking men abducted him from a busy street in Cantonese-speaking Hong Kong and rendered him unconscious. He said they beat him and warned him not to follow through with his plan.

He said they stapled crosses into his thighs because he was Christian. Lam displayed his wounds to reporters at a news conference on Friday, flanked by fellow members of Hong Kong’s Democratic party, before filing a police report and going to a hospital.

Police said on Tuesday they had arrested Lam on suspicion of providing false information to mislead police after investigating his allegations, including checking surveillance footage from cameras in the area.

Howard Lam shows his stapled thighs during a press conference in Hong Kong on Friday. Photograph: Felix Wong/AFP/Getty

The Democratic party chair, Wu Chi-wai, said party leaders decided to air the allegations in public instead of going to the police straight away because they feared for Lam’s safety.

Wu said Hong Kong’s biggest pro-democracy party would provide a full account after the police concluded their investigation. “We hope the truth will come to light as soon as possible,” he said.

Lam’s case has raised concerns that Beijing was tightening its hold on Hong Kong, after other recent cases including the secret detention of a group of Hong Kong booksellers and a Chinese-Canadian tycoon whose whereabouts are unknown.

In both cases, mainland security agents are suspected of taking them across the border, in violation of Hong Kong’s constitution.