HONG KONG (AFP) - Hong Kong's embattled leader on Sunday said "foreign forces" were at work in the pro-democracy movement that has paralysed parts of the city, as demonstrators accused police of using excessive violence during ugly clashes.

Chief Executive Leung Chun-ying said "external forces" from other countries had been encouraging the mass sit-ins that have blocked major thoroughfares for three weeks, but refused to identify them.

"I shan't go into details, but this is not entirely a domestic movement," he said in a television interview on Sunday night.

Chinese state media have repeatedly alleged that "anti-China forces" such as the United States are manipulating the protesters, and Beijing has warned against foreign meddling in what it says is an internal affair.

The rallies come as one of the biggest challenges to Beijing's authority since the Tiananmen pro-democracy protests of 1989.

Leung, described by demonstrators as a pro-Beijing stooge and facing calls for his resignation, said protests had got "out of hand", and called for "a peaceful and a meaningful end to this problem".

He spoke as protest leaders blasted Hong Kong's police force over violent clashes that broke out in the Mongkok district early Sunday, with 20 people injured as officers struck surging crowds with batons.

It was the fourth night of clashes between protesters demanding free elections for the semi-autonomous Chinese city, and police trying to restore traffic to the major Mongkok thoroughfare, which they have brought to a standstill.

The spike in violence comes after three weeks of largely peaceful pro-democracy rallies and road blockades across a financial hub usually known for its stability.

Protesters accused the police of using "deadly" force in Mongkok overnight, with some demonstrators suffering head wounds, fractures and bruising, and others carried away on stretchers.

"If this goes on, one day there may be someone who loses his life or gets seriously injured - then the situation in Hong Kong will get out of control," said activist Lam Cheuk-ting.

Police insisted they had used appropriate force against protesters who attempted to charge their cordon lines.

"Activists from radical organisations as well as troublemakers are mingling with other protesters," said police spokesman Steve Hui.

"From time to time they plan, organise and direct various actions to provoke our officers and create chaos." Four people were arrested overnight, Hui said. Also held was a 23-year-old man who used the Internet to incite others to join the Mongkok protest and to paralyse the railways.

Talks between student protest leaders and government officials are still set to go ahead on Tuesday despite the clashes. But with little common ground, there are slim hopes of a breakthrough.

China insists that candidates for the 2017 vote for Hong Kong's leader must be approved by a pro-Beijing committee - a condition which the protesters dismiss as "fake democracy".

But Leung warned that Beijing has no intention of caving in to the protesters.

He brought along a copy of Hong Kong's mini-constitution to Sunday's interview, saying there was nothing in it specifying the right for Hong Kongers to nominate their own candidates.

Asked if he could rule out a violent crackdown on the demonstrators, he said: "The Hong Kong government and the police force have exercised extreme tolerance and patience. That's what we have done and what we will continue to do." But he also vowed to "restore law and order in Hong Kong as soon as possible".

Although the crowds have shrunk from their peak of tens of thousands earlier in the month, the protests continue to block major roads, with some residents growing increasingly frustrated by the disruption.

Some protesters at the Mongkok camp donned hard hats and makeshift protective gear fashioned out of household items on Sunday in anticipation of further clashes with police.

Posters stuck around the camp read: "Calm down. Don't forget our original purpose." Hong Kong's police force traditionally pride themselves as being "Asia's finest", but their reputation has taken a battering since they used tear gas against the protesters on September 28, with images of the chaotic street battles beamed around the world.

The latest surge in violence comes after video footage emerged last week showing plain-clothes officers beating a handcuffed protester as he lay on the ground.

A former British colony, Hong Kong was handed back to Chinese rule in 1997 under a "one country, two systems" deal that guarantees freedoms not seen on the mainland. But fears have been growing that these liberties are being eroded.