(Updates with new opposition party; South Ossetia incident)

By Margarita Antidze and Matt Robinson

TBILISI, Nov 23 (Reuters) - Georgian President Mikheil Saakashvili called for unity after war with Russia on the fifth anniversary of the street protests that brought him to power, but instead faced a direct challenge from a former ally.

August's war was disastrous for Georgia and Saakashvili, and there was a reminder of the tensions that remain when gunfire broke out at the de facto border of rebel South Ossetia as he and the Polish president approached in a convoy.

There were none of the star-studded celebrations that previously recalled the 2003 "Rose Revolution", which toppled long-time leader Eduard Shevardnadze and brought to power Saakashvili's young, pro-Western government on a promise of democratic reform.

Nino Burjanadze, who co-authored the revolution but has since become disenchanted with Saakashvili, used the anniversary to launch her own party and potentially a bid for the presidency of the former Soviet republic.

Economic liberalisation and growth in the Caucasus state have been overshadowed by accusations of authoritarianism from Saakashvili that critics say has curbed media freedom, compromised the judiciary and stifled opposition.

When Russia intervened in its southern neighbour in August to repel a Georgian assault to retake the Russian-backed breakaway South Ossetia region, Saakashvili's leadership was questioned again, and his opponents were emboldened.

"We mark the fifth anniversary of the Rose Revolution facing great difficulties, facing a very powerful aggressor," Saakashvili said in a televised address late on Saturday.

"We need unity to overcome all these problems, and we need willingness to continue reforms."

Saakashvili remains popular, but is fiercely criticised by those who say he walked into a war that Georgia could not possibly win.

CHALLENGER

The five-day conflict, which strained already difficult relations between Russia and the West, created tens of thousands of refugees and scared off foreign investors, who now look unlikely to return as the global financial crisis takes hold.

On Sunday a convoy carrying Saakashvili and visiting Polish President Lech Kaczyinski towards the de facto border of South Ossetia, which Russia now considers to be independent, had to turn back after gunfire was heard.

Burjanadze split with Saakashvili early this year, citing unfulfilled promises, and is now seen as a potential challenger for the presidency.

"Five years ago, people took to the streets and changed the government. But now we are in a very difficult situation -- we have lost a war, our land is occupied, hundreds died, and Georgia is humiliated," she told a rally launching her party.

"In any civilised country, the government would have resigned or been removed by people." Some 200 opposition activists demonstrated outside the Imedi television station, a former opposition broadcaster shut down by masked police in November 2007 during a brutal crackdown to end days of protests against Saakashvili.

Imedi is back on air, but now toes the government line.

"Freedom of speech in Georgia is under serious threat today, and we're not talking only about Imedi but all television stations," said Kakha Kukava of the Conservative Party.

U.S. President George W. Bush issued a statement on Saturday praising the Rose Revolution, and the U.S. warship Barry anchored off Georgia's Black Sea port of Batumi to take part in a community project. (Additional reporting by Niko Mchedlishvili; Editing by Kevin Liffey)

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