By Josie Cox 

The Russian ruble fell to a 2015 low on Friday, after the country's central bank cut interest rates, catching swaths of forecasters off guard.

The beleaguered currency, which has been struck in recent months by geopolitical tensions, Western sanctions and a relentless tumble in the price of oil, fell more than 2.5% to touch 72 to the dollar after the bank said it was slashing its key rate to 15% from 17%.

That takes the ruble's losses over the past six months against the buck to close to 50%. Since the start of the year it has depreciated close to 14%, making it one of the world's worst-performing currencies.

In a shock move in December, Russia's central bank raised interest rates by 6.5 percentage points to 17%. The bank said Friday's decision was taken because the previous month's increase had been effective in stabilizing inflation.

"The 17% rate is too high for taming inflation risks, driven by monetary-related factors, and not cost-effective," said Oleg Kouzmin, a strategist at Renaissance Capital, adding that the latest move will be positive for the economy and the market.

"Tight monetary conditions put heavy pressures on banks and result in a domestic credit shock, in addition to external [shocks]," he said. He expects the central bank to continue the easing cycle and to bring the key rate gradually back down to 10% by the end of the year.

David Kohl, deputy chief economist at Swiss private bank Julius Baer, however, said it is likely too early to draw conclusions from Friday's move.

"The situation In Russia is still very unstable and it might turn out that the central bank acted too early today. They could come under pressure to reverse the move, or even introduce capital controls," he said.

He does maintain, though, that the ruble has likely reached a bottom against the dollar.

"We see the current level as extremely undervalued and attractive for investment, but wouldn't recommend our clients buy it yet. Political volatility is just too high. What the central bank did today confirms that," he said.

"We certainly live in exceptional times."

Russia's Micex stock index traded broadly steady on the day after the rate- cut announcement. Its dollar-denominated RTS equivalent, however, fell more than 2% before recovering marginally. Over the past half-year, the RTS has lost close to 40% in value.

Strategists said that Russian bond and equities in particular could come under additional pressure in the coming months if rating companies decide to cut Russia's credit rating further.

On Monday, Standard & Poor's Ratings Services notched its rating on Russia down to junk for the first time in more than a decade.

Despite stabilizing somewhat on Friday, Brent crude has fallen almost 55% in value over the past six months, and more than 13% so far in 2015. Barclays on Wednesday slashed its forecast for average Brent prices in 2015 to $44 per barrel from $72.

Goldman Sachs in a note said that its economists don't expect a recovery in prices over the next six months and see risks skewed to the downside in the short term. Oil revenue makes up around 45% of the Russian government's budget.

Elsewhere Friday, European equities were sold off after data showed that consumer prices in the eurozone fell more drastically and more broadly than expected in January.

The European Union's statistics agency said consumer prices were 0.6% lower than in January 2014, having been down 0.2% on an annual basis in December. The decline in prices was the largest since July 2009.

The Stoxx Europe 600 reversed earlier gains to end the session 0.5% lower, while Germany's DAX 30 and France's CAC 30 also slumped into negative territory.

In the U.S. markets sold off too, burdened by figures showed that economic growth in the country retreated to a modest pace in the final months of 2014, underscoring obstacles to the recovery as troubles mount abroad.

Gross domestic product, the sum of all goods and services produced across the economy, expanded at an annual rate of 2.6% in the fourth quarter, the Commerce Department said. Economists surveyed by The Wall Street Journal had expected growth of 3.2%.

Equity markets in Europe have endured a turbulent month, dominated by a ferocious rally spurred by the European Central Bank's announcement of its bond-buying program last week. Ian Williams, economist and strategist at brokerage Peel Hunt said that the "euphoria", however, "has begun to wear off."

In currency markets, the euro was around 0.3% lower against the dollar in late trade Friday at $1.1291 early afternoon. The euro fell sharply in the wake of the ECB's stimulus announcement, but has since steadied.

--Chiara Albanese and Tommy Stubbington contributed to this article

Write to Josie Cox at josie.cox@wsj.com and Tommy Stubbington at tommy.stubbington@wsj.com