Mr Zhvania was a key leader of a popular uprising just over a year ago that swept Mr Saakashvili to power in the impoverished ex-Soviet state. He was the able organiser and trouble-shooter at the heart of the reform movement.
Thousands of Georgians queued from the early morning as snow fell, waiting to pay their respects to the 41-year-old whose death was caused by carbon monoxide from a faulty gas heater on Thursday night.
His coffin was draped in the red and white Georgian state flag and flanked by an honour guard in traditional Caucasus dress.
Many mourners held roses - the symbol of Mr Zhvania and Mr Saakashvili's bloodless revolution.
Some fear Georgia may suffer without his moderating counsel, raising concerns in Western states that are building a key oil export pipeline through the Nato-hopeful country.
Analysts say Mr Zhvania restrained his impulsive president, who will head the government until a replacement prime minister is found, especially in dealings with rebel governments in the regions of South Ossetia and Abkhazia, and in relations with neighbour Russia.
But Mr Saakashvili pledged to stay the course the two men plotted together. "In the face of all the cynics Georgia has become a state. Above all, this was possible thanks to Zurab Zhvania," Mr Saakashvili said in an impassioned speech at the funeral in central Tbilisi.
"I want to say to all who wish ill for Georgia: you have no hope ... [Mr] Zhvania is no longer among the living, but we are still alive. If someone has the fantasy that he can change the business that we all started together, that Zurab Zhvania started, then he is very much mistaken."
The funeral was conducted by Patriarch Ilya II in a new cathedral, the biggest in the country. (Reuters)Reuse content