It comes as defiant protesters have stood their ground outside the city's government headquarters.
The youthful crowds outside the government complex swelled on Saturday night to more than 10,000 people - organisers said there were 60,000 at the peak.
They were protesting against the Chinese government's recent decision to refuse to grant full democracy to the semi-autonomous city, as a tense week of student-led demonstrations came to a climax.
There were scuffles with police overnight as lines of officers pushed back surges of people with riot shields. Many of the protesters wore goggles and wrapped their faces in plastic film in case police used pepper spray in a repeat of angry scenes earlier in the week.
Police made 74 arrests, urging those gathered in the "unlawful assembly" late on Saturday to leave the area as soon as possible.
By Sunday morning only a few hundred weary protesters remained but many of those departing said they would be back after going home to sleep and shower.
Prominent grassroots pro-democracy group Occupy Central has long been planning to amass protesters in the city's financial district.
But in a dramatic speech in the early hours, co-founder Benny Tai said the rally would now merge with the student-led protests outside government headquarters, bringing forward the official launch of the campaign to Sunday from the original start date of October 1.
"Occupy Central starts now," Tai told the crowds.
The group added in a statement "The courage of the students and members of the public in their spontaneous decision to stay has touched many Hong Kong people. Yet, the government has remained unmoved. As the wheel of time has reached this point, we have decided to arise and act.
"The Occupy movement will continue the current occupation."
Beijing dashed the former British colony's hopes for full democracy last month by announcing that, while it will allow elections for the city's next leader in 2017, it will vet the candidates.
The decision sparked a string of protests in Hong Kong, with students going on strike on Monday at the start of a week of angry action that has seen demonstrators mob the city's leader and briefly storm the government complex.
In signs that the protesters are digging in for the long-haul, first-aid stations and recycling points have been set up at the site, with water, snacks and protection from pepper spray freely passed out.
The protests have drawn some support worldwide, with Global Solidarity HK, a coalition of Hong Kong citizens based overseas, organising events in Europe, Canada and the United States, as well as Taiwan.
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