By Margarita Antidze

TBILISI (Reuters) - Georgia's ruling parliamentary coalition stripped President Mikheil Saakashvili of some powers on Thursday, the latest round in a political skirmish that could potentially threaten supplies of Caspian oil and gas to Europe.

The country's billionaire Prime Minister Bidzina Ivanishvili, 57, has been trying to assert his supremacy over Saakashvili since he took office last October, creating an awkward power-sharing arrangement.

The Georgian parliament unanimously passed a motion in the first of three readings to make constitutional changes that water down the president's powers and remove his authority to dismiss the government and parliament.

"I was never going to dismiss the government or the parliament ... But this vote was important for democratic dialogue in our country and for demonstrating that the National Movement is still very strong," Saakashvili, 45, said in a televised statement after the vote, referring to his party.

The prime minister enjoys greater day-to-day authority in Georgia than the president, who has the power to send laws back to the assembly. Until the amendment becomes law, he also will retain the power to dismiss both government and parliament.

The measure was supported by Saakashvili's allies in parliament who had insisted the president had no intention to use his powers against Ivanishvili's Georgian Dream coalition that won elections last October.

Ivanishvili and Saakashvili have cooperated in an uneasy coalition since the Georgian Dream won a majority in parliament last year and political analysts expect further political clashes until the president's term ends in October.

STRATEGIC

Further instability in the country of 4.5 million, which fought a five-day war with Russia in 2008, would worry the West because of the threat to pipelines which carry Caspian natural gas and oil to Europe through Georgia.

The former Soviet republic also commands an important strategic location on the Black Sea between Russia and Iran, Turkey and its oil and gas-producing Azerbaijan.

Some of the president's powers are already due to be transferred to the prime minister under constitutional changes agreed by the last parliament, but Ivanishvili's allies had demanded legislation to ensure Saakashvili could not dismiss the government.

"We don't want Saakashvili to have this stone in his pocket (to throw at the government)," Tina Khidasheli, a lawmaker from the Georgian Dream coalition, said.

The first reading vote took place after a non-binding test vote, in which Saakashvili's allies voted against the measure to prove the Georgian Dream members did not have the two-thirds majority needed to change the constitution on their own.

(Reporting by Margarita Antidze; Editing by Sophie Hares)