Media playback is unsupported on your device Media caption These pictures from witness Cassady Winston show police attempting to secure one side of the main carriageway through the Admiralty district

Police in Hong Kong have clashed with pro-democracy protesters as they cleared an underpass near the government headquarters.

Hundreds of police officers wearing riot gear used pepper spray to disperse the protesters, and arrested dozens of people.

Police said Lung Wo Road had to be cleared as it was a major thoroughfare.

After the scuffles, police said officers involved in beating a protester would be reassigned.

On Wednesday footage aired on local television showed a handcuffed protester being kicked and beaten by police officers during the operation.

Secretary for Security Lai Tung-kwok said later that some officers seen on the video would be removed from their current duties and that an investigation would be carried out.

Demonstrators, a mix of students and a pro-democracy group called Occupy Central, have occupied parts of Hong Kong for more than two weeks. They are demanding fully free elections in the next vote for the territory's leader.

China, which has control over Hong Kong, says residents can vote - but it will vet which candidates are eligible to stand.

Image copyright AFP Image caption Police said in a statement that complaints of excessive use of force by officers would be investigated

Image copyright AFP Image caption Police said they had arrested 45 people on charges of unlawful assembly

Analysis: Juliana Liu, BBC News, Hong Kong

The footage shot by broadcaster TVB, showing a handcuffed demonstrator being punched and kicked by half a dozen police officers for several minutes, has been widely shared on social media.

Accusations of police using excessive force were made when authorities fired tear gas as the protests first erupted in late September. But this incident, which took place at around 03:00 on Wednesday (19:00 GMT Tuesday), was different.

The demonstrator, Ken Tsang, a social worker, had already been detained and no longer posed any threat to law enforcement.

Hong Kong's police force has for years prided itself on its professionalism, political neutrality and experience with crowd control. Now, citizens are asking, why did officers appear to behave with impunity toward a protester who had been subdued?

Media playback is unsupported on your device Media caption Student spokeswoman Yvonne Leung: "The police tactics will only prolong the protests"

The clashes came on the third day of operations that police say are necessary to ease traffic disruption, but which they insist are not aimed at clearing the protesters.

Their new advance came when protesters blockaded the underpass after being cleared out of other areas of the city on Tuesday.

Local television networks broadcast live footage of scuffles, showing police arresting many protesters, whose hands were tied with plastic cuffs.

It was the worst violence seen since the beginning of the protests on 27 and 28 September.

Tsui Wai-Hung, a police spokesman, said 37 men and eight women had been arrested for "unlawful assembly".

None of those arrested had been hurt, the spokesman said. Four police officers were said to have been injured.

Image copyright AFP Image caption Police marched towards protesters blocking the road while holding umbrellas - a symbol of their cause

The protesters are now in their third week of occupying key parts of the city in a bid to put pressure on China and Hong Kong's authorities to answer their calls for reform.

Thousands of people took to the streets at the beginning of the demonstrations but the numbers have dwindled in recent days.

Hong Kong democracy timeline

1997: UK gives Hong Kong back to China under a 1984 agreement giving it "a high degree of autonomy" for 50 years

2004: China says it must approve any changes to Hong Kong's election laws

June-July 2014: Pro-democracy activists hold an unofficial referendum on political reform; both sides hold large rallies

31 August 2014: China says it will allow direct elections in 2017 but will pre-approve candidates

22 September 2014: Student groups launch a week-long boycott of classes

28 September 2014: Occupy Central and student protests join forces and take over central Hong Kong

2017: Direct elections for chief executive due to take place

Q&A: Hong Kong's democracy controversy