Georgians were voting Saturday in municipal elections, a day after ex-president Mikheil Saakashvili's arrest raised the stakes in the polls seen as a key test for the increasingly unpopular ruling party.

The detention on Friday of Georgia's foremost opposition figure upon his return from exile deepened a protracted political crisis in the Caucasus nation.

Founder of Georgia's main opposition party, the United National Movement (UNM) and president between 2003-2013, the Saakashvili, 53, said Friday he had secretly returned from Ukraine, where he heads a Ukrainian government agency steering reforms.

The flamboyant pro-Western reformer, who in 2003 led the peaceful "Rose Revolution" that ousted Communist-era elites, was detained shortly afterwards over a 2018 conviction in absentia on abuse of office charges.

He has denied any wrongdoing, denounced his sentence of six years in jail as politically motivated, and following his arrest went on hunger strike, Georgia's rights ombudsperson said.

His jailing will almost certainly spark upheaval in the small ex-Soviet nation that has been plagued for years by political instability.

"I want to ask you all to go to the elections so that not a single vote is lost," he wrote on Twitter Saturday, posting a picture of a letter to supporters from prison. 

"My freedom and, more importantly, the freedom of Georgia depends entirely on your actions and fighting ability."

They must go

Chairman of Saakashvili's UNM party, Nika Melia, said Saturday the opposition was "winning decisively in the elections, according to the preliminary data from polling stations."

He called for high voter turnout and accused the government of "voter intimidation and vote buying."

"Georgians must be mobilised so that Georgian Dream can't manipulate election results," he told a news conference.

Turnout stood at nearly 18 percent by noon (0800 GMT), the central election commission said.

Standing in a long queue of voters outside a polling station in central Tbilisi, a 39-year-old math teacher, Maya Savaneli, said she "will be voting for chasing Georgian Dream from power."

"I hope Georgian Dream doesn't take much votes today and will be forced to call snap polls."

Another voter, 27-year-old painter Luka Samushia, said: "It will be difficult for the government to falsify vote results if the turnout is high."

"They must to go, they can't jail Saakashvili and remain in power," he said referring to the ruling party.

The municipal elections are being watched inside and outside Georgia for signs of the ruling party Georgian Dream backsliding on democracy.

Saakashvili -- who commands a fiercely loyal following -- called in one video Friday for his supporters to gather on the main thoroughfare in the capital Tbilisi on Sunday to "defend election results."

Referendum against oligarch

Critics have accused Georgian Dream -- in power since 2012 -- of using criminal prosecutions to punish political opponents and journalists. Interpol turned down requests from Tbilisi to issue a red notice against Saakashvili.

Opposition parties decried widespread fraud and refused to take their seats after last October's parliamentary elections, which Georgian Dream won narrowly.

They have staged mass protests, demanding snap polls.

The EU mediated an inter-party agreement in May, under which Georgian Dream pledged to hold a snap parliamentary vote if it wins less than 43 percent in Saturday's local elections.

Georgian Dream withdrew from the pact in July, but the European Union and the United States urged the EU-aspirant country's government to implement the agreement that envisaged sweeping political and judiciary reforms.

Saakashvili insists the deal remains in place, saying the upcoming elections "are a referendum on the removal from power" of the Georgian Dream's oligarch founder Bidzina Ivanishvili. 

Ivanishvili -- Georgia's richest man and a former prime minister -- is widely believed to be calling the shots in Georgia but insists he is no longer a political player.

With concerns mounting in the West over the ruling party's democratic credentials, the United States has hinted at possible sanctions against Georgian Dream officials.