Russia's Bolshoi Theatre finally raises its curtain on Friday after a six year closure for much-delayed reconstruction aimed at restoring its imperial splendour and artistic reputation to their former glory.

The Bolshoi's music director Vasily Sinaisky will lift his baton from the pit at 1500 GMT for the first note in an invitation-only gala opera and ballet concert attended by President Dmitry Medvedev to mark the re-opening.

The historic building hosted its last performance in July 2005 and was then closed for urgent restoration works, without which it risked simply disintegrating with three-quarters of the building deemed to be decaying.

The Bolshoi's entire opera and ballet troupe then moved to a newer but smaller theatre nearby with critics and even its own artists complaining that the cramped stage stifled its epic style.

"One of the main tasks of the reconstruction was bringing in new technology. Before 2005, we were lagging behind Europe by 100 years," Bolshoi director Anatoly Iksanov told the government Rossiiskaya Gazeta newspaper.

"But what we have now after the reconstruction has no rival," he said.

The gala opening, including stars like Romanian opera diva Angela Gheorghiu and the Bolshoi's own dazzling prima ballerina Svetlana Zakharova, is set to be broadcast into 100 cinemas via a live relay and also on YouTube.

They will be treading a stage on which in the halcyon days of the Bolshoi in the Soviet Union appeared legends such as ballerinas Maya Plisetskaya and Galina Ulanova or the mythical male dancer Maris Liepa.

The wide broadcast of the event is some consolation for fans who would have loved to have witnessed the reopening at first hand but never had the chance.

Iksanov said invitations were sent out only by the Kremlin administration and not by the theatre itself. Thus the 1,740 lucky enough to have seats will be the elite and not ordinary Russians.

"Could you imagine what the reaction would be if London's Royal Opera House gave up the entire theatre to the presidential administration?" leading music critic Marina Gaikovich told the Interfax news agency.

"Here in Russia, as always, everything is closed and secret," she added.

The restoration has been mired in controversy from the start, overshooting the original budget over four times and missing its original reopening date of 2008 as the state of the building was found to be far worse than first believed.

"Usually in such cases people don't do repairs but demolish," said Mikhail Sidorov, a representative of the Russian building company Summa Capital, which took over the project in 2009.

But the results are spectacular and replicate as faithfully as possible how the theatre looked when it was rebuilt by Russian-Italian architect Albert Cavos in 1856 after a fire.

Restorers removed the Soviet coat of arms from the facade, replacing it with the double-headed eagle, the Tsarist symbol readopted by Russia. The Soviet hammer and sickle is also gone from the curtain.

Restorers spent three years replacing red silk wallpaper in the so-called Imperial Foyer, opened in the late 19th century to celebrate the coronation of the last tsar, Nicholas II.

Yet there are also huge changes in an effort to improve the acoustic. The number of seats has been reduced from the previous 2,155 and a new hall has been built underground for chamber concerts.

Artistic hopes are also high with the Bolshoi already staging innovative new productions of great operas and seminal works of modern dance on the second stage that has been its home for the last decade.

Most notably, US ballet star David Hallberg has joined the Bolshoi for the new season, the first time an American has become a member of the legendary company whose stars until now were all Russian or from ex-Soviet states.

The first theatrical performance will be on November 2 of Glinka's "Ruslan and Lyudmila" -- seen as Russia's national opera -- in a production directed by Dmitry Chernyakov.