* Part of $1.5 bln loan will go on servicing previous debt
* Lukashenko expected to return to Putin for more
* Putin can't afford to see ally toppled
By Gabrielle Tétrault-Farber and Katya Golubkova
MOSCOW, Sept 15 (Reuters) - A $1.5 billion loan from Russia will shore up Belarusian President Alexander Lukashenko for the time being as he tries to face down mass protests, but not for long.
Lukashenko, whose security forces have arrested thousands of opposition supporters demanding his resignation, secured the financial lifeline from Russian President Vladimir Putin at a Black Sea summit on Monday.
But the actual cash available to him - for example, to pay his police force or raise salaries to lure back striking workers - will be much smaller than the headline figure.
According to Artyom Shraibman, a political analyst at the Carnegie Moscow Center, only a fraction of the funds will actually make it to Minsk, given it needs to refinance another $1 billion in loans, including from Russia, and pay $300 million of debt to Russian energy giant Gazprom by the end of the year.
"In reality this is money being transferred from one Russian pocket to another," said Pavel Latushko, a prominent member of the Belarusian opposition council.
Sofia Donets, chief economist at Renaissance Capital in Moscow, said: "The loan is basically not new money, but the refinancing of the existing debt of Belarus (to Russia), part of which is repayable next year."
Almost half of the total external public debt of Belarus is due to Russia, she said. It faces external debt payments of about $2.6 billion next year, mainly split between Russia and China.
Besides facing the gravest political crisis of his 26-year rule, Lukashenko is in deepening economic trouble.
Gross Domestic Product fell 1.3% between January and August, the official news agency Belta said on Tuesday. In August alone, the central bank burned through $1.4 billion, or about a sixth of its gold and foreign exchange reserves, as it fought to shore up the rouble currency.
Belarusian companies and banks are also highly exposed. Russian credit ratings agency ACRA said they had external debt of about $9 billion as of July 1, and the government may end up shouldering some of that because of their limited scope to reschedule during the current crisis.
ACRA estimates Belarusian debt to Russia at around $7.9 billion, or 12-13% of GDP as of April 1 this year.
Lukashenko has relied heavily for the past two decades on Russian financial support for Belarus's stagnating economy. By some estimates he has received more than $100 billion over that period, mainly in oil and gas subsidies.
He has taken advantage of the fact that Russia needs its small neighbour as a buffer against NATO and an export route for its oil and gas. And Putin has no desire to see another neighbouring leader toppled by demonstrators, as happened with the 'Maidan' protests in the Ukrainian capital Kyiv in 2014.
Putin "has to support him, because if he doesn't, this regime will collapse," said political analyst Dmitry Oreshkin.
"For the Kremlin this is a very dangerous example of what could be used in Russia," he added. "People will see that it's possible to change leaders with mass protests."
For that reason, the fates of the two presidents are bound together, analysts said.
Although Lukashenko would have wanted more funds from Moscow, both he and Putin got something important from Monday's encounter, said Ryhor Astapenia of London's Chatham House think-tank.
"Lukashenko is saved, economically speaking, for at least some time," he said. "And Putin feels sure that Russian influence will increase in Belarus." (Additional reporting by Vladimir Soldatkin and Tatiana Voronova Editing by Mark Trevelyan and Peter Graff)