Tens of thousands of people thronged the streets of Georgia's capital on Sunday to show their opposition to President Mikhail Saakashvili in the largest anti-government demonstration in three years.

The protest was seen as a test of public support for the opposition ahead of a parliamentary election in October.

It also was a political coming-out party for organizer Bidzina Ivanishvili, a billionaire businessman who is Georgia's richest man and leading philanthropist. He made his entry into politics in October, announcing that he was forming a political party with the aim of winning the parliamentary vote and assuming the post of prime minister.

Saakashvili's second and last presidential term ends in January, and his future plans are unclear. He has not excluded becoming prime minister, a position that will gain additional powers under a 2010 constitutional reform that his opponents said was designed to allow Saakashvili to remain a political force after leaving the presidency.

But this would bring unwelcome comparisons to Russian President Vladimir Putin, who spent the past four years as prime minister to accommodate a constitutional ban on two consecutive presidential terms.

Organizers estimated Sunday's turnout at 110,000, while police put it at at least 30,000.

As the participants marched from three directions to join the rally on Freedom Square, they carried Georgian flags but also the flags of the European Union and NATO, indicating that support remains high for the goals set under Saakashvili for Georgia to one day join the Western organizations.

The demonstrators also wrote their "dreams" on pieces of paper and stuffed them in sacks printed with the name of Ivanishvili's new party: Georgian Dreams.

"My dream has always been a strong and united Georgia, where people live freely and are paid what they are worth," Ivanishvili said to the crowd. "I waited for Georgia to become strong and united, but the transitional period after the gaining of independence dragged on and I decided to begin to fight this regime."

Georgia won its independence with the collapse of the Soviet Union in 1991 and began moving closer to the West when Saakashvili became president in January 2004. He is credited with pushing through a series of political and economic reforms, but he also led the country into a disastrous war with Russia in 2008 over two breakaway Georgian republics now fully allied with Moscow and beyond Tbilisi's control.

Saakashvili weathered weeks of opposition demonstrations in 2009 demanding his resignation over his handling of the war, but the splintered opposition groups failed to coordinate and the protests fizzled out.

Ivanishvili has formed a coalition with some opposition parties but has refused to work with others, including the party led by veteran opposition leader Nino Burdzhanadze.

Sunday's rally was opened by Kakha Kaladze, the former captain of Georgia's national soccer team who played for nine seasons with Milan, and it ended with a performance by Ivanishvili's eldest son, Bera, a rap musician.