The announcement to slash its key interest rate to 15 percent from 17 per cent marked the first cut since December 2011.
The rate cut had come after the government urged the central bank to help boost the economy, prompting some analysts to accuse the bank of caving in to political pressure.
The ruble tumbled following the announcement, falling to 81 against the euro and 71 against the dollar. A day ago, it was trading at 77.6 to the euro and 68.73 to the dollar.
It added that it had taken action now "due to the shift in the balance of risks of accelerated consumer price growth and cooling economy".
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Russia's economy is expected to contract at an annual rate of 3.2 percent in the first half of 2015, the bank said.
Output growth reached 0.6 percent for 2014, official data showed Friday, nearly half of the 1.3 percent reached in 2013.
Economists had largely predicted the central bank to hold the rate in order to keep a lid on inflation, although it slowed investment in the economy and bankers and industrial leaders were calling for a cut.
Although some economists backed the move, others accused the central bank of caving to political pressure.
"We generally welcome such a central bank decision and consider it positive for the economy and the market," said Oleg Kuzmin of Renaissance Capital in a research note. He said the 17 percent rate was too high and caused a "domestic credit shock."
Ermolenko predicted that any effect on the economy would be negative: weakening the ruble and increasing capital flight due to lack of confidence in Russia's economic policy.
First Deputy Prime Minister Igor Shuvalov on Thursday criticised the central bank as "obsessive" in stressing its "independence from the ruling authorities."
The central bank had raised rates six times last year in a bid to counter the negative impact of sanctions and falling oil prices.