Georgia claimed today to have foiled a Russian-backed plot to stage a military coup on the eve of joint exercises with Nato troops.

An Interior Ministry spokesman announced that the special services had uncovered a plot to topple the Government involving a former high-ranking officer at the Defence Ministry.

A tank battalion had mutinied at a base in Mukhrovani, 20 miles east of Tbilisi, and was refusing to obey orders, the Defence Ministry said.

The dramatic development comes a day before Georgia hosts three weeks of military exercises with Nato that Russia has condemned as provocative so soon after the war over the breakaway region of South Ossetia in August last year.

Mikheil Saakashvili, the Georgian President, is also facing a sustained protest campaign from opposition parties demanding his resignation.

David Sikharulidze, the Defence Minister, told Georgian television that several hundred troops in the tank battalion had launched a rebellion aimed at overthrowing the authorities and wrecking the Nato exercises.

"The mutiny is continuing. The rebellious battalion was told to stop," Mr Sikharulidze said.

The base had been sealed off and talks were continuing, but the mutineers had made no concrete demands. The base is home to the Gori tank battalion.

Tanks were leaving Tbilisi and heading towards the base held by the mutineers, according to the Georgian news website Civil.ge. Helicopters were seen flying over the capital.

An Interior Ministry spokesman, Shota Utiashvili, said that special services had exposed a wider conspiracy to launch a coup involving several army units. A former commander of a special forces unit had been arrested.

"The preliminary investigation materials show that the plot was co-ordinated with the Russians and was aimed at disrupting the Nato training," Mr Utiashvili said. "They were receiving money from Russia. It seems it was co-ordinated with Russia."

The uprising was linked to discontent over the political situation in Georgia, he added. Opposition parties began street demonstrations on April 9 to force Mr Saakashvili, but support for the protest has been dwindling.

Russia's ambassador to Nato, Dmitri Rogozin, dismissed as "mad" Georgian claims of Moscow's involvement in the coup attempt. He told Interfax: "It can be said that both the Georgian army and Georgian nationhood are undergoing complete destruction, and the reason is again Saakashvili's mad policies."

Doubts were also expressed from inside Georgia. Giya Karkarashvili, an opposition leader and former defence minister, said: "Today Georgia is in the hands of sick people, who write the scenario themselves, play it themselves, then make a movie and show it to people for intimidation purposes."

Nato declined to comment. About 1,000 soldiers from more than a dozen member states and partners are due to practise "crisis response" exercises at Vaziani army base, east of Tbilisi and about 44 miles (70 km) from the nearest Russian troop positions in South Ossetia.

Russia assumed operational responsibility last week for border security in South Ossetia and Georgia's other breakaway region of Abkhazia. It has stationed 7,800 troops in the two regions, although the Georgian authorities claim that there are at least 10,000.

President Medvedev described the exercises as "an open provocation" and warned: "All responsibility for possible negative consequences will lie with the people undertaking the relevant steps."

The alleged rebellion follows a resumption of formal contacts between Nato and Russia last week, the first since the war in South Ossetia. Tensions were already running high after Nato expelled two Russian diplomats in a spying scandal, including the son of Moscow's ambassador to the European Union.

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