The attempt will add to fears that Russia is backing attempts to turn parts of the Balkans away from greater integration with the west, in part by supporting ideologues and militant nationalist groups, FT reported.
Montenegro has become a "testing ground for a geopolitical confrontation" between Russia and the west, Milan Knezevic, an MP and one of the alleged plot leaders, told the Financial Times.
Mr. Knezevic and Andrija Mandic, another opposition MP, are accused by Montenegro's state prosecutor of forming a criminal group linked to the plot to stage terrorist attacks on election day last October.
"We will organize a popular, consultative referendum that will show most people oppose NATO," Mr. Knezevic said at his party offices, adorned with Orthodox icons and oil paintings of Marshal Georgy Zhukov, Josef Stalin's military chief.
Instability in the country of fewer than 1m people is one of a number of security concerns in the Balkans preoccupying EU leaders as Moscow steps up its efforts to regain influence in the region. The situation is "fragile," said Donald Tusk, president of the European Council. "The EU will remain engaged in the western Balkans and stand by its commitments."
Read alsoRussia behind attempted Montenegro coup – prosecutorMontenegrin authorities say they have evidence that Russian "nationalistic structures" — including officials — orchestrated October's attempt to provoke street clashes, storm the parliament and assassinate Milo Djukanovic, the former prime minister.
Moscow has described the official account as "absurd," while Mr. Knezevic and Mr. Mandic say the allegations are a politically motivated ploy to imprison opposition leaders. Both men lead parties allied with the United Russia party of Vladimir Putin.
Mr. Mandic separately confirmed his support for an unofficial poll on NATO membership if the government refused a formal referendum on joining the security alliance.
The opposition MPs' demand echoes the use of illegal referendums by pro-Russian separatists three years ago in Crimea and eastern Ukraine to undermine that country's sovereignty.
Montenegrin leaders have wavered in their allegiance between Russia and the west since the 1999 war, but have pursued EU and NATO membership since regaining independence in 2006. In 2008, the government bolstered its pro-western credentials by recognizing Kosovo's unilateral declaration of independence from neighboring Serbia.
Montenegro's government plans to use a vote in parliament to approve plans to join NATO. Accession to the alliance is then expected to be confirmed at a NATO summit in May.
Read alsoMontenegro seeks arrest of Russian spy over coup plot - The TimesHowever, the Netherlands, Spain and the U.S., which are NATO members, need to ratify Montenegro's membership before the May summit. The effort has aroused skepticism among many citizens who remember NATO's bombing campaign against Slobodan Milosevic, then Serbian leader.
"They bombed us in 1999 and now they won't allow our people to vote on whether we join NATO or not?" asked Marko Milacic, a pro-neutrality campaigner who welcomes Russia's "return to the Balkans" as a corrective rebalancing after years of western tutelage.
Mr. Knezevic said the result of a referendum would "be a basis to change the government and, after May, to hold a formal referendum to take us out of NATO."
Montenegrin authorities said it was unclear how advanced the opposition's plans were for the unofficial poll, or if they represented an idle threat. But one official said he believed the aim was to provoke a police crackdown and possibly violent clashes.
Mr. Knezevic said: "The government will use all means to prevent this: I don't rule out arrests, closing down polling stations and the full repressiveness of the regime."
Mr. Djukanovic said MPs had legal channels through which to request a referendum and authorities should prevent an unauthorized poll. "Their wish is to demonstrate violence," he said, adding that Russia's policy was to push back the region's Euro-Atlantic integration.
"The purpose of the attack was to send a message to NATO, saying you can't continue enlarging as you once planned — not only in Ukraine and Georgia but also in the Balkans."
Read alsoSerbia to get Russian fighter jets amid Montenegro intention to join NATOAnalysts said NATO opponents had drawn encouragement from the U.S. presidential election and Russia's increased focus on the Balkans but were unlikely to succeed in blocking membership. Western diplomats agreed the planned poll posed little risk to Montenegrin accession. "It's a done deal," said one ambassador.
Officials from three major NATO allies confirmed that their governments thought the October plot was orchestrated by the Kremlin. Some believed the intention was to deny NATO control of the strategically important Adriatic port of Bar.
At least two Russian men are accused of organizing the coup attempt from neighboring Serbia on behalf of Russia's GRU intelligence service, before leaving Belgrade for Moscow.
Another 21 suspects are in police custody in Podgorica awaiting indictment. The supreme state prosecutor has denied the special prosecutor's request to detain Mr. Knezevic and Mr. Mandic before they are indicted formally.
Srdjan Darmanovic, Montenegro's foreign minister, said: "We have pretty active and very non-hidden opposition of Russia to our... full-fledged NATO membership. Russian officials have never hidden that."