Hong Kong police have used tear gas to disperse pro-democracy protesters outside the main government building, after a week of escalating tensions.
Demonstrators trying to push through police barricades were earlier repelled by pepper spray.
Protesters want the Chinese government to scrap rules allowing it to vet Hong Kong's top leader in the 2017 poll.
Hong Kong Chief Executive CY Leung said the demonstration was "illegal" and elections would go ahead as planned.
In his first public statement since the protests began, Mr Leung also added that consultations would continue.
He said he and his government had "been listening attentively to members of [the] public". But, he said, "resolute" action would be taken against the "illegal demonstration".
Protesters blocked a busy road on Sunday, clashing with police as they tried to join a mass sit-in outside government headquarters.
Police used hand-held cans of pepper spray to drive back the protesters, who defended themselves with umbrellas and face masks.
As evening fell, the police lobbed tear gas canisters into the crowd, scattering some of the protesters.
The BBC's Juliana Liu in Hong Kong described chaotic scenes in the streets around the main government complex.
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Chief Executive CY Leung said the government was "resolute in opposing the unlawful occupation" by protesters
Despite the tear gas and pepper spray, she said, the large crowds did not appear to be dispersing, moving instead into a park adjacent to the complex.
Thousands joined a sit-in outside government headquarters this weekend, bolstering a week-old protest, which began as a strike by students calling for democratic reforms.
On Saturday night, the leader of Occupy Central, another protest movement, brought forward a planned action to merge it with the sit-in by the students outside the central government building.
A statement by the movement said Mr Leung had "failed to deliver on political reform".
The protesters had also called for further talks but it is not clear how far - if at all - Mr Leung's mention of further consultations will be seen as recognising their demands.
Faith Kwek, a 19-year-old student protester, said Mr Leung's "words are just words".
"I don't think myself or any of the protesters will give in until we see bigger progress in the form of action from him. We don't want our country to surrender to China."
Occupy Central had originally planned to paralyse the central business district next Wednesday, but organisers advanced the protest and changed the location in an apparent bid to harness momentum from student protests outside the government complex.
Student activists had stormed into a courtyard of the complex late on Friday and scuffled with police using pepper spray.
Police said they made more than 60 arrests including prominent student activist leader Joshua Wong.
The BBC's Juliana Liu says that thousands had arrived spontaneously to support the demonstration by students.
Those outside the government buildings plan to stay until they are forcibly removed, she adds.
Police protect a cordon outside government offices in Hong Kong. 28 Sept 2014
Overnight, police protected a cordon outside the government offices
However, some students expressed unease that their protest was apparently being taken over by Occupy Central.
"A lot of students left as soon as Occupy made the announcement they were starting their occupation," said university graduate Vito Leung, 24.
"I think they were really forcing it. This was always a separate student movement with similar goals but different directions. I don't think it should be brought together like this."
Unrest began when the Chinese government announced that candidates for the 2017 chief executive election would first have to be approved by a nominating committee.
Activists have argued that this does not amount to true democracy.
At least 34 people have been injured since the protests began, including four police officers and 11 government staff and guards, authorities said.