Montenegro police fired tear gas to disperse thousands of protesters demanding the resignation of PM Milo Djukanovic.
Demonstrators in the country’s capital, Podgorica, chanted “Milo, thief!” and demanded fair elections organized by a transitional government.
Some threw flares and fireworks at police guarding parliament.
Fifteen policemen were injured and 24 protesters sought treatment after being tear gassed, the interior minister said.
A leader of the Democratic Front opposition group, Andrija Mandic, was been questioned by police after the rally, along with another protest leader, Slaven Radunovic.
Milo Djukanovic has been in power for much of the past two decades, after holding prominent positions in the republic from the early 1990s when it was part of the former Yugoslavia.
In 2012, he won elections and became prime minister for the third time since Montenegro became independent after splitting from neighboring Serbia in 2006.
Montenegro’s government hopes to be invited to join the NATO in December, but many Montenegrins with historic ties to Russia are opposed to this.
The country’s general elections are scheduled for early 2016.