TBILISI (Reuters) - Mariam Tsertsvadze, the co-founder of an animal charity in the Georgian capital, has managed only three or four hours of sleep a night in the past few weeks, but says she is full of energy.

Tsertsvadze has joined thousands of other Georgians in nightly protests in the centre of Tbilisi to voice their opposition to a government bill on "foreign agents" that critics have condemned as authoritarian and Kremlin-inspired.

The bill, which parliament approved in the second of three readings on Wednesday, would require organisations receiving more than 20% of their funding from abroad to register as agents of foreign influence.

Tsertsvadze has a direct stake in whether the bill passes. She's the co-founder of an animal welfare organisation called Animal Project, which provides support to stray dogs and cats in Tbilisi.

The group has received funding from Western donors and is looking to increase its foreign support. She fears the legislation would undermine its work.

At home on Thursday evening, Tsertsvadze went about what has become her normal nightly routine: gathering her gas mask, goggles, eye drops and other essentials into a black rucksack.

At previous protests, police had cleared crowds with tear gas, stun grenades and water cannon. Eleven people, including six police officers, had been hospitalised the night before. Tsertsvadze wanted to be prepared.

Petting her dog and cat goodbye and placing a whistle around her neck, the 33-year-old braved the rain and headed out for another night of protest.

"Either you are on the right side of history or on the wrong side of history," Tsertsvadze said outside the parliament building after greeting friends "I strongly, strongly believe that I am on the right (side)."

'ISOLATED FROM THE WESTERN WORLD'

The draft law has sparked a rolling political crisis and drawn comparisons to laws used to suppress dissent in Russia, with whom Georgia fought and lost a short war in 2008.

The ruling Georgian Dream party says the bill will promote transparency. Tsertsvadze said suggestions that Georgian non-profits are controlled by foreigners are "demeaning".

"I fear that (the bill) will completely change the foreign (policy) course of Georgia," she said. "We will be left completely isolated from the Western world".

The standoff is part of a wider struggle that could determine whether Georgia, a country of 3.7 million people that has seen turmoil, war and revolution since the collapse of the Soviet Union, moves closer towards Europe or back under Moscow's influence.

After first gathering outside the parliament, Tsertsvadze and thousands of other protesters - schoolchildren to pensioners draped in Georgian and EU flags - poured into Tbilisi's central Heroes' Square.

She says she and her fellow protesters are determined to continue until the end.

If the bill passes, she said, "we will be left completely alone (to cope) with our government.

"Our job right now is to protest".

(Reporting by Reuters in Tbilisi; Writing by Lucy Papachristou in London; Editing by Ros Russell)

By Felix Light and Ali Kucukgocmen