The arrested men planned to pick up automatic weapons to attack state institutions, police and possibly officials, police chief Slavko Stojanovic said.
One Serb was on the run, he said.
The election is taking place amid tension over plans to forge closer ties between Montenegro and the West.
The arrested Serbs have been charged with “forming a criminal organisation and terrorism”, Mr Stojanovic said.
Polls predict the closest race since independence and there are fears of violence between opposition and government supporters after the result is announced.
Under Prime Minister Milo Djukanovic the country is pursuing membership of the EU and Nato.
But his Democratic Party of Socialists is being challenged by opposition groups, some of which are pro-Serbian.
Mr Djukanovic has presented the vote as a choice between EU integration and falling under Russian influence, but the opposition has denied his allegations that it received funding from Moscow.
However many Montenegrins remain angry that Nato bombed Serbia and Montenegro in 1999 as part of a strategy to halt the killing and expulsion of ethnic Albanians in Serbia’s then southern province of Kosovo.
The country has also seen an influx of Russian money, homebuyers and tourists since splitting from Serbia in 2006.