* Ivanishvili nominates Interior Minister Garibashvili

* PM says his work done after Saakashvili loses presidency

* Saakashvili favoured West over Russia

* Prime minister most powerful man in country

By Margarita Antidze

TBILISI, Nov 2 (Reuters) - Georgia's prime minister named Interior Minister Irakly Garibashvili as his successor on Saturday, handing a close ally the task of treading a political tightrope between Russia and the West.

Billionaire Bidzina Ivanishvili has said he will quit as premier after Georgy Margvelashvili is sworn in as president on Nov. 17.

Margvelashvili was elected last month to take over from Mikheil Saakashvili, who spent a decade in power pursuing friendly relations with the West - often at the expense of Russia.

Ivanishvili, who entered politics two years ago following a business career in which he made a fortune estimated at $5.3 billion, made efforts to maintain good ties with the West while improving them with Moscow.

He named his successor at the headquarters of his Georgian Dream coalition.

Under changes to the constitution that will this month transfer many of the president's responsibilities to the prime minister, the 31-year-old Garibashvili will become the country's most powerful politician.

He said on Saturday he would stick with the outgoing premier's policies.

"I promise our people, our country that I'll continue the policies and economic development (plans) started by Bidzina Ivanishvili," Garibashvili said.

BEHIND THE SCENES

Garibashvili's task will be complicated by the state of ties between Russia and the West, strained by tensions over the Middle East as well as Moscow's granting of asylum to fugitive U.S. intelligence contractor Edward Snowden, and the detention of 30 Greenpeace activists.

Ivanishvili, whose party's dominance of parliament has allowed him to eclipse Saakashvili politically over the past year, succeeded in opening the Russian market for Georgian wine, mineral water and fruits after a ban imposed in 2006, two years before the two countries fought a five-day war.

Garibashvili still has to be nominated by parliament and approved by Margvelashvili, but both steps are viewed as a formality.

Margvelashvili's election cemented Georgian Dream's grip on power, putting it in charge of the presidency for the first time, as well as the government and legislature, which it took control of in a parliamentary election in October last year.

Ivanishvili says the departure of Saakashvili, a bitter rival, means his job is now done.

But he is expected to continue wielding influence behind the scenes in the South Caucasus country of 4.5 million, which is crossed by pipelines that carry Caspian oil and gas to Europe.