Earlier in the day, the Montenegrin parliament approved changes to the law, which limits powers of the country's president to give out a mandate to form a government, after a lengthy debate in a 41-81 vote.
The protesters claim the changes to the law violate the country's constitutional order, according to the Radio and Television of Montenegro. They broke through a security fence, after which the police resorted to using tear gas, the report said. Some demonstrators also reportedly threw stones and other objects at police officers. The crowd chanted 'Betrayal, betrayal,' the broadcaster said.
The Montenegrin Interior Ministry said that at about 05:30 p.m. local time (16:30 GMT) the authorized part of the rally ended, and some protesters used tear gas against police officers.
"Part of the crowd went to a security fence in front of the Montenegrin Assembly building and forcibly removed some of it. Then there was an organized attack on police officers at the cordon, with protesters throwing stones, bottles, pyrotechnics and other means of mutilation, as well as spraying tear gas in the direction of the officers and the parliament," the police department reported.
Later in the day, protesters promised to block traffic in all Montenegrin cities on Tuesday, according to the Radio and Television of Montenegro.
"Starting tomorrow, we will block off all cities in Montenegro. How many of us will be there, and there are enough of us, you will see," the broadcaster quoted protester Predrag Vushurovich as saying.
The protest is headed by the movement "Ima Nas," or "There are plenty of us," which is associated with the political party Patriotic Komitas Union of Montenegro and is close in ideology to the Democratic Party of Socialists of President Milo Dukanovic.
The Montenegrin leader refused to sign the first draft of the law forcing him to entrust the formation of a government to his political opponent Miodrag Lekic from the liberal conservative Democratic Alliance (DEMOS). Dukanovic also announced his readiness to dissolve the parliament, which caused a wave of discontent among lawmakers.