It came a day after his alleged accomplice also told Hong Kong's High Court that he was assaulted in custody.

The two men have been charged over the attack on Kevin Lau, former editor of the investigative Ming Pao newspaper, in a brutal street stabbing in broad daylight in February last year before escaping on a motorcycle.

The attack intensified fears over press freedom in Hong Kong.

Defendants Yip Kim-wah and Wong Chi-wah, both 39, were detained in mainland China before being returned to Hong Kong in mid-March last year.

They deny charges of malicious wounding with the intent to cause grievous bodily harm.

Wong on Friday claimed he was assaulted on multiple occasions while in police custody in China after refusing to admit to allegations that he and Yip were involved in the wounding case.

"They hit my face and my chest and abdomen" with their fists, said Wong.

"They kept asking me to confess," he said.

"They told me the central government placed great emphasis on this case, and... they must have someone to admit it," he said.

Wong added he was also physically attacked by Hong Kong officers who collected him from the border with China, saying he was slapped and hit.

Yip, in his testimony on Thursday, said he too was beaten into confessing to the crime.

But in cross-examination prosecution lawyer Nicholas Adams asked Wong why a picture of Lau was found on his mobile phone.

"You've got this photo of Lau in your phone so you could identify him when you went to chop him," Adams suggested.

Wong, who said he was nowhere near the crime scene at the time of the stabbing, said the photo had been planted on his phone by Chinese police.

Earlier in the trial, 50-year-old Lau, who was stabbed six times, recalled how he was attacked by "a hard object" before a motorcycle carrying two men sped off.

The attack came just weeks after Lau was removed from his position at the helm of the Ming Pao and replaced with an editor deemed to be pro-Beijing.

His sacking triggered staff protests and widespread fears that Beijing was tightening control of the press in the semi-autonomous territory.

Hong Kong was a British colony until it was handed back to China in 1997 and is ruled under a "one country, two systems" deal.

The system allows it far greater civil liberties than those enjoyed on the Chinese mainland, including freedom of speech and the right to protest.