Hong Kong police on Wednesday carried out a major exercise in the Police College in Aberdeen to prepare for potential conflict during a mass sit-in organized by the Occupy Central movement scheduled for Tuesday.

The exercise included scenarios such as demonstrators pushing through police defensive lines and possible clashes between police and protesters, according to local media reports.

Hong Kong-based Ming Pao reported that some 50 police vehicles arrived at the Police College at around 7 am Wednesday morning.

Participants in the drill include mobile units from various districts, emergency units and criminal investigators.

At around 10 am, some "protesters" were seen holding banners and marching forward while chanting slogans. The "protesters" were stopped by police, and then the two sides "clashed."

According to previous media reports, police have been preparing for the sit-in, an operation dubbed Solarpeak, since last year.

Some 3,000 police officers will be deployed to maintain order during the protest, which will go ahead if the plan for universal suffrage for the 2017 chief executive election fails to meet the demands of the protesters, zaobao.com reported.

The South China Morning Post, however, disputed the number and said some 4,000 police officers will be deployed.

A source close to the Hong Kong police implied they may move in forces from other districts to deal with Occupy Central. He told the Global Times that criminal cases might increase at the time due to weakened patrols in other parts of Hong Kong.

The Police College will be turned into a temporary detention center during the protest.

The Occupy Central movement has staged a "referendum" which asked voters to choose from three shortlists, all of which suggest that any electoral plan for the chief executive should start with public nominations instead of Beijing's plan for a nominating committee.

The referendum has been repeatedly rebuffed by Hong Kong and the central government as illegal and invalid.

Separately, China on Thursday voiced opposition to the US State Department's remarks.

"Hong Kong's political system is a domestic affair in which other countries have no right to interfere," said foreign ministry spokeswoman Hua Chunying, after spokeswoman of the US State Department Marie Harf said, "the legitimacy of the chief executive will be greatly enhanced if ... the election provides the people of Hong Kong a genuine choice of candidates representative of the voters' will."