HONG KONG (Reuters) - There is no legal way for a Hong Kong bookseller who returned to the city following detention on the mainland to be sent back to China, Hong Kong's security chief said on Wednesday.

The comments by Secretary for Security Lai Tung-kwok came after a one-day trip to Beijing to meet officials and discuss the case of five Hong Kong booksellers who went missing last year, before surfacing in mainland Chinese custody.

One of the booksellers, Lam Wing-kee, returned to Hong Kong last month and held a news conference detailing months of captivity and repeated interrogations by mainland agents.

A mainland Chinese law enforcement body on Tuesday said Lam had violated his bail terms and criminal enforcement measures would be triggered by his failure to return, but did not specify the measures.

Stepping up an official propaganda campaign against Lam, Chinese state television CCTV on Wednesday night broadcast images of the bookseller confessing during his detention.

"I very much regret that I have violated the provisions of Chinese law," Lam said. "I hope the Chinese government can treat me leniently after this incident, because I am certain I will never cause a situation like this again."

A formal request for Lam's return to China had not been discussed at the Beijing meeting, Lai said.

"There is no legal arrangement for the transfer of a person to the mainland authorities and the Hong Kong government will handle all cases in accordance with the law of Hong Kong," Lai told reporters on Wednesday.

Hong Kong officials pledged to provide any necessary police protection for Lam, who said he had been followed by strangers and feared for his safety. The officials said the vehicle that was reportedly following Lam was rented by the media.

Hong Kong police met Lam again on Wednesday, taking a further statement from him during a 2-1/2-hour discussion.

Hong Kong is a special administrative region of China, governed by separate, and significantly freer, laws under a 1997 pact that returned the former British colony to Chinese rule.

The controversy over the Hong Kong booksellers erupted last year when the five men associated with the Causeway Bay Books store disappeared. The store had specialized in gossipy books about China's leaders, including President Xi Jinping. Such books are banned on the mainland, but legal in Hong Kong.

Two of the men, Gui Minhai and Lee Bo, held Swedish and British passports and went missing in Thailand and Hong Kong, respectively.

Thousands have taken to the streets in Hong Kong following the disappearance, saying Beijing has not abided by the "one country, two systems" arrangement by which it agreed to rule Hong Kong for 50 years.

Hong Kong officials said they would meet mainland counterparts at the end of July to discuss the notification process for mainland detention of Hong Kong residents.

(Reporting by Sharon Shi, Hera Poon and Venus Wu in HONG KONG and Megha Rajagopalan; Writing by Clare Baldwin; Editing by Greg Torode and Clarence Fernandez)