MOSCOW (Reuters) - Former Beatle Paul McCartney said on Thursday he had written to Russian President Vladimir Putin to enlist his help in securing the release of a group of Greenpeace activists detained in Russia.

Twenty-eight activists and two journalists face up to seven years in jail over a protest against offshore drilling in the Arctic in which some members of the environmental group tried to scale a Russian oil rig.

"It would be great if this misunderstanding could be resolved and the protesters can be home with their families in time for Christmas. We live in hope," McCartney wrote on his website.

He said Mr Putin, whom he met when he first performed in Moscow in 2003, had not replied to the Oct 14 letter, although he said Russia's ambassador to London responded by saying the group's plight was not properly represented by the media.

"Vladimir, millions of people in dozens of countries would be hugely grateful if you were to intervene to bring about an end to this affair," McCartney wrote in the letter.

Kremlin spokesman Dmitry Peskov said he only heard of the letter from media reports but had not received it, according to the state-run Itar-Tass news agency.

The Prirazlomnaya oil rig which the Greenpeace activists tried to scale on Sept 18 is owned by state energy giant Gazprom and is at the heart of a Kremlin drive to tap the Arctic's natural resources and expand Russia's economy.

Quoting from the Beatles' song Back in the USSR, which he composed 45 years ago, McCartney wrote: "That song had one of my favourite Beatles lines in it: 'Been away so long I hardly knew the place, gee it's good to be back home.'"

"Could you make that come true for the Greenpeace prisoners?" he asked.

Investigators charged the 30 with hooliganism, to replace original charges of piracy, which carry a 15-year jail term.

Greenpeace says piracy charges have not been formally dropped and that the 30 are now effectively charged with both.

The Netherlands, where Greenpeace has its main headquarters, has asked an international tribunal to order the release of the group from pre-trial detention.

McCartney has previously expressed support for the Pussy Riot protest group, which performed a "punk prayer" in a Moscow cathedral last year against Putin's ties with the Russian Orthodox Church.

Appeals for leniency failed to help them escape a two-year jail sentence.

Critics accuse Mr Putin of using the courts to persecute his opponents, a charge the Kremlin denies.