* Russian stock indexes down 1.5-1.6 pct
* RTS index hits lowest level since September 2009
* Rouble up v. dollar but down v. euro, supported by interventions (Adds comments and details, updates prices)
MOSCOW, March 13 Russian stocks slid further on Thursday, before a referendum in Crimea that investors fear could bring harsh Western sanctions against Russia.
At 1400 GMT, the dollar-denominated RTS index had fallen 1.5 percent to 1,084 points. The rouble-denominated MICEX was down 1.6 percent at 1,254.
The RTS index fell to its lowest since September 2009 and MICEX to its lowest since May 2012. The indexes have lost 13-14 percent since the beginning of the month.
"The further down (the market) goes, the more people sell because they get margin calls," said one Moscow-based trader. "People are scared that U.S. funds will be forced to sell because of sanctions. (They are scared) the U.S. is going to implement tough sanctions that will hurt the economy and cut Russia off from the market."
Renaissance Capital trader Alexei Bachurin said stocks rose briefly on Thursday morning because investors were buying to cover short positions, bets the market would fall.
"There is nowhere to make sales," he said. "For now there are such pessimistic expectations that people are defending themselves."
The Moscow-backed parliament in Ukraine's Crimea has voted in favour of joining Russia. Residents of the region will vote on Sunday in a referendum on whether to secede from Ukraine and join Russia. Most Western countries say the vote is illegal.
On Thursday, German Chancellor Angela Merkel warned of "massive" political and economic damage and a "catastrophe" if Russia proceeded with the annexation of Crimea. EU foreign ministers are meeting on Monday to discuss further sanctions against Russia.
The international stand-off over Ukraine is also expected to curb the growth of Russia's economy, which was already performing poorly.
On Thursday, Goldman Sachs cut its forecast for Russian GDP growth this year to 1 percent from 3 percent, citing tension in Ukraine and an acceleration in capital outflows. The bank estimates a net $45 billion to $50 billion of capital has left Russia so far this year and says the flow for the full year may reach $130 billion, double last year's.
Individual stock performances were mixed. Major losers included leading bank Sberbank, which fell 3.8 percent, and airline Aeroflot, down 7 percent. But VTB bank was up 1 percent and gas giant Gazprom was down just 0.2 percent.
The rouble weakened slightly, supported by central bank interventions that kept the rouble within a floating corridor against a dollar-euro basket.
The rouble firmed 0.2 percent against the dollar to 36.43 on stronger oil prices but gave up 0.2 percent versus the euro to trade at 50.84.
The Russian currency was down 0.1 percent at 42.95 against the dollar-euro basket that the central bank uses to guide the rouble's nominal exchange rate.
The central bank shifted the rouble's corridor by 10 kopecks as of Wednesday, to 35.90-42.90. That implies the rouble was trading on Thursday at a level where the central bank carries out unlimited interventions to support it.
The bank releases its intervention data with a two-day lag, but VTB Capital estimates the bank spent about $2 billion on Wednesday to prop up the rouble.
"More hawkish headlines might follow the Crimea referendum on Monday, which is likely to fuel some more rouble weakening," VTB Capital analysts said in a note. "Nevertheless, we think the central bank needs to address it with an increase of forex interventions."
For rouble poll data see
For Russian equities guide see
For Russian treasury bonds see
Russia in graphics: link.reuters.com/dun63s (Reporting by Jason Bush, Megan Davies, Lidia Kelly and Olga Popova; Editing by Larry King)