"If the referendum is avoided and there is a bid to fraudulently pass the decision (on NATO membership) in parliament, Montenegro will be brought to the verge of a (civic) conflict," Andrija Madic, the leader of pro-Russian opposition New Serb Democratic Party, told the crowd.

The protest outside parliament gathered between 2,000 and 5,000 people, according to estimates by reporters and organisers and pro-Russian and pro-Serbian opposition parties.

On December 2 NATO invited Montenegro to become its 29th member, defying Russia's opposition to what Moscow branded a threat to its security.

Montenegro's veteran Prime Minister Milo Djukanoy has firmly rejected calls to organise the referendum on acceding to the North Atlantic Treaty Organization.