HONG KONG — A mob of masked men opposed to Hong Kong’s pro-democracy demonstrators led an apparently coordinated assault on the protest zone in the heart of the city’s financial district Monday, tearing down barricades and clashing with police.

The chaotic scenes came after police carried out a dawn operation to reopen some key roads blocked by protesters for more than 15 days. Police said they will continue to chip away at the occupied zone to relieve traffic, and warned that anyone who challenges them could be arrested.

Monday’s confrontations highlighted the growing tension between student-led protesters and authorities — as well as other residents aggravated by the disruptions. The protesters, who had enjoyed widespread support when the movement began, are fighting to keep up momentum as the political crisis entered a third week.

Demonstrators have flooded several thoroughfares in central Hong Kong since Sept. 28 in a civil disobedience movement to oppose restrictions on the first-ever direct election for the semiautonomous Chinese city’s leader, promised by Beijing for 2017. They want authorities to drop a plan to use a pro-Beijing committee to screen candidates, and demand the resignation of Chief Executive Leung Chun-ying, the city’s Beijing-backed leader.

Authorities have repeatedly urged protesters to retreat from the streets, but student leaders have vowed to keep up the disruptions until the government responds to their demands.

Protesters were rebuilding barriers Monday after about two dozen men wearing surgical masks to hide their faces tore at the metal barricades that demonstrators have put in place. The men, who used box cutters to snap the cable ties connecting the barricades, scuffled with protesters and police who tried to stop them.