Georgia's prime minister resigned on Thursday over plans to arrest a top opposition leader, saying it risked escalating a political crisis in the ex-Soviet nation.

Giorgi Gakharia said he was stepping down because of disagreement in the government over enforcing the court order to arrest opposition leader Nika Melia.

-It is inadmissible to enforce a judiciary decision... if that poses a risk to the health and lives of our citizens or creates the possibility of a political escalation in the country.-

A court in Georgia on Wednesday ruled to place Melia in pre-trial detention.

Melia, who faces up to nine years behind bars if found guilty of "organising mass violence" during anti-government protests in 2019, has rejected the charges as politically motivated.

The move to arrest Melia -- chairman of the country's main opposition force, the United National Movement (UNM) -- raised the stakes in a political crisis that has gripped Georgia since parliamentary elections in October.

The opposition denounced the polls as rigged after the ruling Georgian Dream party claimed victory.

Leaders of nearly all of the country's opposition parties gathered Wednesday at the UNM party headquarters in the capital Tbilisi ahead of the trial, vowing to obstruct police if they moved to arrest Melia.

Georgian television stations on Wednesday night aired footage of riot police mobilising close to the UNM headquarters.

In a statement ahead of the trial, the European Union envoy to Georgia described the circumstances surrounding Melia's prosecution as a "dangerous trajectory for Georgia and for Georgian democracy."

The US embassy in Tbilisi said on Twitter that the crisis "must be resolved peacefully", urging both the government and the opposition to show restraint.