Hong Kong police have cleared one of the largest protest sites that has choked the city for months, arresting scores of pro-democracy activists in what could be a turning point in the fight to wrest greater political freedom from Beijing's control.

Riot police clashed with protesters late into the evening as activists sought to regroup and regain lost ground.

Student leaders Joshua Wong and Lester Shum were among those arrested as hundreds of officers swept through the bustling area of Mong Kok, clearing barricades and tents that had blocked key roads in the Chinese-controlled city for more than two months.

Some among the pockets of demonstrators still out on the streets denied the setback marked the beginning of the end of the occupation, and it was not clear if or when police might try to remove the remaining protest sites elsewhere in the city.

Scuffles broke out when riot police moved against hundreds of protesters on Nathan Road in gritty Mong Kok, Reuters witnesses said.

"You can't defeat the protesters' hearts!" screamed Liu Yuk-lin, a 52-year-old protester in a hard hat holding a yellow umbrella, the symbol of the movement, as she stood before lines of police in helmets and goggles.

But there was no serious violence, and after about three hours the operation was complete and traffic was flowing through the area where demonstrators had camped out since late September to call for greater democracy in the former British colony.

Mong Kok has been a flashpoint for clashes between students and mobs intent on breaking up the protests, which have posed one of the biggest challenges to China's Communist Party leaders since the crushing of student-led pro-democracy demonstrations in Beijing in 1989.

Crowds nearby cheered and clapped as the final protesters were removed from the site on Wednesday.

Earlier, court-appointed bailiffs had warned protesters to leave and around 80 workers in red caps and "I Love Hong Kong" T-shirts began clearing metal and wooden barricades laid across Nathan Road, where hundreds of tents had been erected in a two-month civil disobedience campaign.

They had been met by hundreds of protesters brandishing yellow banners and chanting for "full democracy".

"If you resist you face possible imprisonment.

"We warn you to immediately stop resisting," said a policeman into a loud hailer before jeering activists.

More than 100 people have been arrested in Mong Kok over the past two days.

Hong Kong's Cable TV said 4,000 police were involved in yesterday's operation.

A Reuters witness saw police take away Shum, and the Facebook page of the student group Scholarism announced that Wong had been arrested for contempt of court.