An outspoken Hong Kong pro-democracy media mogul was attacked on Wednesday by a group of men who threw bags of rotten animal organs at him, according to his spokesman and witnesses.

Jimmy Lai owns Next Media -- which runs the Apple Daily newspaper and website, often publishing articles critical of Beijing -- and had been visiting the city's main pro-democracy protest site in the Admiralty business district.

"They threw pig intestines at him. I think it was three guys," his spokesman, Mark Simon, told AFP adding that 66-year-old Lai had escaped unhurt.

Fears over the erosion of press freedom in Hong Kong have grown following attacks on journalists earlier this year.

Former editor of liberal newspaper, Ming Pao, Kevin Lau, was stabbed in broad daylight by two men in February.

Apple Daily's headquarters have been targeted during the democracy protests by unidentified anti-Occupy groups shouting and blocking distribution lorries.

Bundles of the newspaper have also been doused in soy sauce at distribution points.

Witnesses told how three men arrived carrying a suitcase and started throwing stinking plastic bags filled with putrid brown animal organs at Lai on Wednesday afternoon, as one of them shouted: "Go die".

"(It) was evident Lai was the target of the three men... (the bags) hit his head and his body," Alex Kwok, one of the volunteer marshalls who safeguards protesters at the Admiralty site, told AFP.

"We marshalls immediately tried to stop them. Afterwards, some citizens came to help and three started beating the marshalls including myself," added Kwok, who had a light bruise under his left eye.

Two men were taken away by police, an AFP reporter at the scene said.

Another man was lying on the ground, his wrists and ankles secured with plastic cable ties, the reporter said.

Several major roads have been blocked for more than six weeks by pro-democracy protesters calling for fully free elections for the city's leader in 2017.

Beijing has insisted that candidates must be vetted by a loyalist committee, in what protesters brand "fake democracy".

Simon said that Lai, who is a regular visitor to the Admiralty protest site, would not be deterred from showing his support in future.

"He will definitely go back (to the site)... It's not that big of a deal for Jimmy," he said, adding that Lai had already decided not to press charges.

"These guys are hired. They earned some money today. Why waste time on those clowns?"

Simon did not want to speculate on who was behind the attacks.