Hong Kong pro-democracy newspaper tycoon Jimmy Lai was arrested and released by police Wednesday after turning himself in, the latest figure targeted in a broadening investigation which protest leaders describe as harassment.

Tension remains high in the semi-autonomous southern Chinese city after more than two months of street blockades by demonstrators seeking free leadership elections. Lai's home and office were firebombed last week and he was also targeted during the protests.

The rallies ended in December when protest camps were cleared, but police have vowed to investigate the "principal instigators".

Lai, 66, whose company produces the outspoken Apple Daily newspaper, had been asked to report to Wanchai district police headquarters Wednesday.

He spoke briefly to reporters after more than two hours inside.

"I have been arrested," he said, after his release.

He added that he had been accused of two offences, "organising and assembling an illegal assembly, and participating in an illegal assembly".

Asked if police would take further action, he said: "I don't know. How can I know? They have the right to call me back anytime."

Police later said that Lai had been arrested for organising and participating in an "unauthorised assembly" and had been released after refusing police bail.

"Police reserve the right for prosecution... at this stage (there are) no court proceedings," a police spokeswoman told AFP.

A number of prominent protesters including student leaders Joshua Wong and Alex Chow have voluntarily reported to police stations after being requested to do so. They were all placed under arrest for protest-related offences, but released hours later.

Both Chow and Wong questioned the process, saying police should charge them if they had the evidence.

"If they have enough evidence, for sure... they can lay the charges," Chow said after his release Sunday.

"I can't think of any other reason (for the arrests) other than creating white terror or a politically motivated one," he said.

Wong said Friday police had told him the investigation was still ongoing.

But his lawyer Michael Vidler said authorities were trying to create a "sense of uncertainty" and that if charges were laid at a later time based on evidence already in hand it would constitute "an abuse of process".

Lai was a regular visitor to the protest camps and was arrested during the police clearance of the main Admiralty site.

He stepped down as chairman of Next Media in December following his arrest, but is still a major shareholder.

Lai was targeted during the rallies by a group of men who threw rotten meat at him. The headquarters producing the Apple Daily were also repeatedly blockaded to disrupt distribution.