Hong Kong's leader said Monday that pro-democracy protests were "in vain" after police used pepper spray and batons on students trying to storm government headquarters overnight, in some of the worst violence since the rallies began.

With the protests now into their third month and frustrations mounting, Chief Executive Leung Chun-ying hinted that further police action may be imminent, in his most forceful comments in recent weeks.

"I have pointed out before that Occupy Central is not only illegal but it will also be in vain," Leung said, describing the continued protests as "intolerable".

"Now the (public) demand for police clearance is increasing. From now on, police will enforce the law without hesitation," he told reporters.

But the leader of the student group spearheading the pro-democracy movement declared Sunday night's action a success.

"The government headquarters was paralysed this morning... to a certain extent, the goal of the action was achieved," Alex Chow of the Hong Kong Federation of Students told demonstrators Monday at the main Admiralty protest site.

The government offices were closed on Monday morning and the city's legislature suspended after protesters broke through police lines and occupied a major road outside the complex overnight.

Authorities said they had "no other choice" but to use pepper spray and batons to force them back.

By Monday morning crowds had been driven back to the nearby Admiralty protest site, where they voiced their fury.

"I feel angry but there's nothing we can do about it," said account clerk Justin Yan, 22.

"They (police) are supposed to protect the citizens, not (hurt) us. We saw what they did so we don't trust them any more."

Police arrested 40 people and 11 officers were injured, a spokesman said. Authorities said a total of 37 people received hospital treatment.

Protesters began staging mass sit-ins on major roads at three locations on September 28, demanding free leadership elections for the semi-autonomous Chinese city in 2017.

China's communist authorities insist candidates for the election must be vetted by a loyalist committee, which the protesters say will ensure the election of a pro-Beijing stooge.

Demonstrators now fear that police will try to clear Admiralty, where hundreds of tents block a multi-lane highway through the heart of the financial district.

- 'Angry and tired' -

There was frustration and pessimism at the Admiralty site Monday.

"We feel a mixture of things: angry, tired, upset. All the emotions are quite negative and tense," said student Eppie Chan.

Some protesters said police had attacked them for no reason.

"We were in the front line this morning but we were not attacking... the police came out and hit us," said social work student Joanne Tsang.

Others voiced their doubts over the direction of the movement and Sunday night's call to storm the government HQ.

"The crowd was not prepared for the battle last night... they were put into a difficult situation and didn't know how to handle it," said translator Mayson Ng.

"They (student leaders) don't have a strategy."

The Admiralty site had calmed by Monday afternoon after a chaotic morning which saw protesters at a nearby shopping arcade clash with police.

The protests drew tens of thousands of people at times during their first weeks, but the numbers have dwindled as the movement's leaders struggle to keep up momentum.

Frustrations have grown amongst the demonstrators as Beijing refuses to budge on the vetting of candidates, while support for them has waned among residents weary of the transport disruption.

Police cleared a protest site in the Mongkok district last week, making more than 140 arrests, but sporadic scuffles have continued there.

A smaller camp blocks another busy road in the shopping district of Causeway Bay.

A British colony until 1997, Hong Kong enjoys civil liberties not seen on the Chinese mainland, including freedom of speech and the right to protest.

But fears have been growing that these freedoms are being eroded under Chinese rule.