The protesters are said to have parked trucks filled with gravel and other heavy vehicles at roads leading to the Jarinje and Bernjak border crossings.
The police should therefore have closed the border controls and urged citizens to choose other routes.
During the protests, shots are also said to have been fired, but no one is said to have been injured, writes Reuters.
Serbian license plates
Fourteen years after Kosovo declared independence from Serbia, around 50,000 Serbs living in the northern parts of the country still use Serbian number plates on their cars.
The government has given the Serbs 60 days, starting August 1, to get license plates from Kosovo.
From August 1, all Serbian citizens visiting Kosovo also need special documents to cross the border, writes Reuters.
Similar protests took place almost exactly a year ago.
Similar rules
Similar rules apply to those traveling from Kosovo to Serbia.
Kosovo has been recognized as an independent state by more than a hundred countries - but not by Serbia or Russia.