Hong Kong demonstrators clashed with dozens of masked men at the main protest site in the city centre as police struggled to contain the chaos, according to reports.
Masked men rushed barricades at the Admiralty site as protesters tried to guard the barriers and push the men back, according to the AFP news agency.
The Associated Press news agency reported that the men charging the protesters numbered in the hundreds.
Two of the men were tackled to the ground by police, who also formed a cordon around the masked group as protesters shouted: "Weapons! Weapons!" and "Arrest the triads".
Several hundred people, many of them middle aged or older men, gathered in front of the barricades on a main road Monday, chanting, "Open the road!''
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They were held back by a line of police officers. Some people in the crowd tried to remove the metal barricades that protesters have set up to block off main roads near the heart of the city's financial district.
They also shouted, "Occupy Central is illegal,'' referring to one of the names of the protest movement that has swept Hong Kong.
Al Jazeera's Divya Gopalan, reporting from Hong Kong, said that protesters used loudspeakers to announce that there may be as many as one thousand of their opponents mingling with them in the crowd.
"Anti-protesters, we are told, are members of construction and workers unions, the taxi union and members of the general public that are just fed up of these protests," Gopalan said.
Police took away some masked men inside the protest zone who tried to pick fights with the protesters.
Police remove barricades
Hong Kong police had earlier removed some barricades used by the protesters, who have been occupying main roads in the Asian financial centre for two weeks.
Officers took away unmanned metal barricades at the edges of the city's Central financial district early on Monday, ahead of rush hour.
Police said they did not want to clear the entire protest zone but that, instead, their aim was to relieve congestion and reduce the chance of traffic accidents.
The student-led protesters are occupying a business area as well as streets in two shopping districts.
They are calling on the government to drop plans to use a pro-Beijing committee to screen candidates in the territory's inaugural 2017 election for top leader.
Gopalan said that the territory's leader, CY Leung, who has the title Chief Executive, was in China on Monday attending a trade fair.
"He was asked if there was going to be a crackdown and he refused to answer that question, so it's pretty much a wait and see situation," she said.