Hundreds of Hong Kong police officers drove protesters from an underpass in the dead of night in the worst violence since the street demonstrations for greater democracy began more than two weeks ago.

Officers, many with riot shields and wielding pepper spray, dragged away dozens of protesters, tore down barricades and removed concrete slabs the protesters used as road blocks around the underpass.

Beijing issued its harshest condemnations yet of the protests, calling them illegal, bad for business and against Hong Kong's best interests.

That appeared to reflect Beijing's increasing impatience over the demonstrations in the semi-autonomous Chinese territory, although there were no immediate signs the central government was planning to become directly involved in suppressing them.

A front-page editorial in the People's Daily, the ruling Communist Party's mouthpiece, condemned the protests and said "they are doomed to fail".

It said: "Facts and history tell us that radical and illegal acts that got their way only result in more severe illegal activities, exacerbating disorder and turmoil.

"Stability is bliss, and turmoil brings havoc."

The operation came hours after a large group of protesters blockaded the underpass, expanding their protest zone after being cleared out of some other streets.

The protesters outnumbered the police officers, who later returned with reinforcements to clear the area.

The underpass borders the city government headquarters and is a short walk away from the main protest zone straddling a highway on the opposite side of the complex.

Demonstrators appeared to storm the tunnel in reaction to police attempts over the past two days to chip away at barricades on the edges of the sprawling protest zone.

Officers took away many activists, their hands tied with plastic cuffs, and pushed others out to a nearby park.

Police said they had to disperse the protesters because they were disrupting public order and gathering illegally.

They arrested 45 demonstrators during the clashes, which police said injured four officers.

"I have to stress here that even though protesters raised their hands in their air it does not mean it was a peaceful protest," said the spokesman, Tsui Wai-Hung. He said some protesters kicked the officers and attacked them with umbrellas.

None of those arrested was hurt, he said.

But local television channel TVB showed footage of around six plain-clothes police officers taking a man around the side of a building, placing him on the ground and kicking him.

Local politicians and activists identified the protester as Ken Tsang, a member of a local pro-democracy political party who in 2012 interrupted chief executive, Leung Chun-ying's inauguration by heckling then-Chinese president Hu Jintao.

Secretary for security Lai Tung-kwok later said the officers who were involved have been reassigned and the police department is carrying out an investigation.

The student-led protesters are now into their third week of occupying key parts of the city to pressure the Asian financial centre's government over curbs recommended by Beijing on democratic reforms.

They oppose plans for a pro-Beijing committee to screen candidates to run in Hong Kong's first direct elections to choose a leader, called a chief executive, in 2017. They also want Mr Leung to resign.

When negotiating the 1997 handover of Hong Kong from Britain, China's ruling Communist leaders agreed to a "one country, two systems" that would preserve Western-style civil liberties and broad autonomy in the territory, while promising eventual democracy.

Mr Leung has said there is "almost zero chance" that China's government will change its rules for the election.

Police have chipped away at the protest zones in three areas across the city by removing barricades from the edges of the areas they are occupying.

Positions on both sides have been hardening since the government called off negotiations last week, citing the unlikelihood of a constructive outcome given their sharp differences.

The demonstrations have posed an unprecedented challenge to the government.

Organisers say as many as 200,000 people thronged the streets for peaceful sit-ins after police used tear gas on September 28 to disperse the unarmed protesters. The numbers have since dwindled.

Beijing is eager to end the protests to avoid emboldening activists and others on the mainland seen as a threat to the Communist Party's monopoly on power.