Hundreds of Hong Kong pro-democracy protesters are locked in a tense standoff with police who are warning they will crack down on a mass civil disobedience movement outside government headquarters in the former British colony.

The protesters have been camped out on the streets outside the government complex all weekend. Students started the rally but by early Sunday leaders of the broader Occupy Central civil movement said they were joining them to kick off a long-threatened mass sit-in.

The protesters reject Beijing's recent decision to restrict voting reforms for the first-ever elections to choose the city's leader in 2017.

Police have warned those involved in what they call illegal gathering to leave the scene as soon possible or else they would start clearing the area and making arrests.