(Adds Saakashvili to testify on war on Friday)

TBILISI, Nov 27 (Reuters) - Georgia must follow through with reforms promised after war with Russia in August if it is to weather the coming economic storm and limit social discontent, an influential think tank said.

The Brussels-based International Crisis Group, in a briefing issued late on Wednesday, said that although pro-Western President Mikheil Saakashvili's position was for now secure, "his administration will be severely tested politically and economically in the winter and spring ahead".

Georgia stands to lose international goodwill and some of the $5.4 billion of aid promised after its five-day war with Russia if Saakashvili does not expand and implement a promised wave of democratic reforms, the ICG said.

Saakashvili's young government came to power in the former Soviet republic on the back of the 2003 "Rose Revolution", but has disappointed its Western backers with the slow pace of democratic reform.

Economic liberalisation and growth have been overshadowed by Saakashvili's perceived authoritarianism that critics say has stifled the media, judiciary and opposition and concentrated power in the hands of the president.

The opposition accuses Saakashvili of walking into a war Georgia could not possibly win. The president will face questions about the events leading up to and during the war when he testifies before a parliamentary commission on Friday, his spokeswoman said.

Russia intervened to repel a Georgian assault on breakaway South Ossetia, driving the Georgian military from the country's two pro-Russian rebel regions. Moscow has since recognised both as independent states.

Tens of thousands of Georgian refugees have been unable to return home, and foreign investment has plummeted.

"The global economic crisis and flight of foreign investment are likely to increase social discontent, but it remains unclear who will mobilise these grievances," the ICG said.

"Tbilisi must restore stability to encourage foreign investment and development."

The briefing paper called for more effective social assistance, a truly independent judiciary, elimination of high-level corruption, increased freedom for the broadcast media, changes to the electoral code and transfer of some presidential powers to the legislature and government.

Georgia's former ambassador to Moscow angered the government this week when he contradicted the leadership and told the parliamentary commission he believed Georgia started the war, wrongly convinced that it had U.S. blessing.

Officials said on Thursday that Saakashvili's testimony had been postponed, but later said it would go ahead on Friday. (Writing by Matt Robinson; Editing by Sophie Hares)

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