HONG KONG (Reuters) - A prominent member of Hong Kong’s Democratic Party said on Friday he was beaten and “stapled” by mainland agents in the Chinese-controlled city before being dumped on a beach in what activists said was the latest warning to the democracy movement.

Democratic Party member Howard Lam shows off his injury at a news conference in Hong Kong, China August 11, 2017. Apple Daily via REUTERS

Howard Lam, a key pro-democracy activist in the former British colony, said he was even told in a telephone call not to give a photo signed by Barcelona footballer Lionel Messi to the widow of Chinese dissident Liu Xiaobo.

Lam was confronted by men speaking Mandarin, spoken in Beijing but not widely in Cantonese-speaking Hong Kong, outside a sports store, he told reporters.

He said the men took him away, interrogating him and stapling his skin 21 times for being “unpatriotic” in a nine-hour ordeal. He was knocked out and eventually found himself dumped on a beach in Hong Kong’s remote Sai Kung district.

“This is either to warn off the people of Hong Kong or create problems between Xi and Hong Kong,” said democracy activist Lee Cheuk-yan, referring to Chinese President Xi Jinping. “We do not know the whole objective of the beating up. Both may be the case.”

Hong Kong became a “special administrative region” of China in 1997, since when it has been governed under a “one country, two systems” formula that guarantees a range of freedoms not enjoyed in China, including a direct vote for half of the 70-seat legislative assembly.

But activists say those freedoms have come under threat with perceived meddling by Communist Party rulers in Beijing.

Hong Kong’s police commissioner, Lau Wai-chung, told media he was taking Lam’s accusations seriously and they were attaching great importance to investigating the case.

The Hong Kong and Macau Affairs Office of China’s State Council was not immediately available for comment.

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Chinese authorities have repeatedly said they would never do anything illegal and that Hong Kong’s autonomy was fully respected.

Mainland officers cannot enforce Chinese law in Hong Kong. If Chinese agents were found to be involved, the case is expected to send ripples through the local and diplomatic community about the Chinese state’s willingness to flout both its own and international laws.

In an exclusive interview with Reuters less than two weeks after she was sworn in as Hong Kong leader on July 1, Carrie Lam said it was her obligation to stand up for the city’s core values if she thought they were being undermined.

Lam has pledged to unify Hong Kong at a time when public resentment swells over Beijing’s perceived growing interference in its affairs.

In July, Hong Kong’s high court expelled four pro-independence lawmakers from the city’s legislature after invalidating their oaths of office, in what was seen as the clearest indication of direct intervention by Beijing.

The 2015 abduction of several Hong Kong booksellers, who sold publications critical of China’s leaders, by mainland agents also shook confidence in Beijing’s promise of non-interference, activists say.

Howard Lam said he had received a call from a Chinese person claiming to be part of the mainland intelligence service. He said he was warned not to give the Messi photo to the widow of Liu Xiaobo, the imprisoned Chinese Nobel laureate who died from cancer last month.

It was not immediately clear how they knew of his plans to do so.

Pro-democracy lawmakers, academics and political activists worry that Hong Kong is becoming more like mainland Chinese cities, where the internal security services join forces with police to crush dissent.

Xi swore in Hong Kong’s new leader last month with a stark warning that Beijing won’t tolerate any challenge to its authority in the city as it marked the 20th anniversary of its return from Britain to China.