Protesters demanding the government's resignation and early parliamentary polls rally in front of the parliament in Tbilisi

Up to 20,000 opposition supporters rallied in Georgia on Monday, stepping up pressure on the increasingly unpopular ruling party, despite a police crackdown last week and the arrest of protesters.

Waving Georgian and European Union flags, and chanting "Change!" and "Resign!" protesters marched on the capital Tbilisi's main thoroughfare before gathering outside the parliament building.

Opposition leaders said at the rally that mass protests will continue until their demands for electoral reforms, the government's resignation and snap legislative elections are met.

"Georgian people and the entire opposition spectrum is trying to force the ruling party to introduce a fair electoral system in the country," the leader of opposition Republican Party, Levan Berdzenishvili, told AFP.

"Mass protests will continue until our demands are met," he added.

Braving sub-zero temperatures, hundreds of protesters -- who were mostly young -- stayed at the protest overnight, many dancing to electronic music.

They blocked entrances to parliament, vowing to prevent lawmakers from entering the building where a plenary session is set to be held on Tuesday.

In the early hours on Tuesday, police arrested five protesters and briefly detained prominent opposition MPs, Giga Bokeria of the European Georgia party, and Tengiz Gunava of Georgia's main opposition force, the United National Movement.

"The government has lost popular support and only relies on police force," one of the protesters, Elene Mikadze, 21, told AFP.

"That means its end is coming."

Pro-opposition Mtavari TV station reported that riot police has been deployed in numbers in parliament's underground compound and in the prime minister's office.

The demonstrators have held a series of mass protests after MPs from the ruling Georgian Dream party voted down on November 14 legislation to hold parliamentary elections next year under a new proportional voting system.

Opposition parties have called the rallies after forming a rare united front against Georgian Dream led by oligarch Bidzina Ivanishvili, whom they accuse of orchestrating the bill's failure.

- Rejected compromise -

Protesters say that the current voting system unfairly favours the ruling party, which won nearly 77 percent of seats in the 2016 parliamentary election despite garnering only 48.7 percent of the vote.

Georgian Dream has ruled out early polls and on Monday rejected the opposition's new compromise initiative of legislative amendments that would create a level playing field for all the political forces in the tiny Black Sea nation.

Last Monday, riot police used water cannons to disperse protesters and arrested several dozen people outside the parliament building.

Ten demonstrators have been jailed for terms ranging from four to 13 days for "disobeying police orders".

Georgia's public defender said at the time that the prosecution of the peaceful protesters "failed to meet minimum judicial standards."

In a joint statement last week, the embassies of the United States and European Union criticised Georgian Dream's failure to introduce the electoral reform and expressed solidarity with the protesters.

Ivanishvili had promised "large-scale political reform" following a summer of protests that saw 240 people injured in a police crackdown.

Two protesters including a teenage girl lost an eye.

In power since 2012, the ruling party has seen its popularity plummet amid widespread discontent over economic stagnation and perceived backsliding on its commitment to democracy.

Critics accuse Ivanishvili -- who is widely believed to be the man in charge in Georgia -- of persecuting political opponents, suffocating critical media, and creating a corrupt political system where his private interests dominate government decision-making.