Police stand guard during a protest at the New Town Plaza shopping mall in Shatin in Hong Kong on December 15, 2019.

Clashes broke out between protesters and police in Hong Kong on Sunday, cutting short a rally after thousands had gathered at a park to call for electoral reforms and a boycott of the Chinese Communist Party.

Police fired tear gas near the park, known as Chater Garden, after some protesters attacked men whom they believed to be plainclothes officers, in a return to the violence that has roiled the Chinese territory off and on for months.

Sporting their movement's trademark black clothing and face masks, rally participants had earlier packed into Chater Garden, located near the city's Legislative Council building. They held up signs that read "Free Hong Kong" and waved American and British flags.

"We want real universal suffrage," the protesters chanted. "Disband the police force, free Hong Kong!"

Ventus Lau, the rally's organizer, was arrested in the evening for allegedly breaching the authorities' conditions for the rally and repeatedly obstructing officers, police officer Ng Lok-chun told reporters at a news briefing.

Earlier in the day, Lau said he believes more large-scale protests are needed for global attention to return to Hong Kong, with the protest movement losing some of its momentum in recent weeks.

"I think Hong Kong has not been the focus of the world anymore," he said, urging other countries to launch sanctions against Hong Kong's government if it does not allow residents to directly elect Legislative Council members this year.

A former British colony, Hong Kong was returned to China in 1997. While the framework of "one country, two systems" promises the city greater democratic rights than are afforded to the mainland, protesters say their freedoms have been steadily eroding under Chinese President Xi Jinping.

Frictions between democracy-minded Hong Kongers and the Communist Party-ruled central government in Beijing came to a head last June, when proposed extradition legislation sparked months of mass demonstrations.