(Recasts with closing levels; adds details)
* FTSEurofirst 300 loses 3.2 pct, down 11 pct in a month
* $255 bln wiped off the market value of STOXX 600 stocks
* Greek shares, bonds most hammered in sell-off
* Correction is 'buying opportunity' -Amundi's Boscher
By Blaise Robinson
PARIS, Oct 15 (Reuters) - A sell-off in European stocks accelerated on Wednesday, with a key index suffering its biggest one-day slide in nearly three years as investors slashed exposure to risky assets on mounting worries about global growth.
The slump represented a wipe-off in market value of about $255 billion for European stocks listed on the broad STOXX Europe 600 index. That is more than Portugal's GDP and more than the entire market capitalisation of Europe's biggest oil company, Royal Dutch Shell.
Shares extended their slide in afternoon trading after data showed U.S. retail sales declined in September and prices paid by businesses fell, fuelling concern that consumer demand may be faltering while inflation is failing to gain traction.
Greek equities were among the biggest losers, as Athens's benchmark ATG index succumbed to a second day of selling pressure and sank 6.3 percent. Traders cited political uncertainty and a spike in Greek 10-year bond yields, which rose above 7.6 percent.
"There's been a big acceleration of the sell-off in stocks, with a spike in risk aversion spreading across the board to bonds and the currency market, and even a return of stress around Greek assets," said Alexandre Baradez, chief market analyst at IG France.
"The newsflow is quickly deteriorating, including today's U.S. data. It's nothing to reassure investors. All the ingredients are there for further losses.
"In this 'risk-off' swing, global investors are dumping their most risky holdings, and obviously Greek stocks and bonds fall in this category."
The FTSEurofirst 300 index of top European shares ended 3.2 percent lower at 1,251.87 points, a level not seen since last December. It was the benchmark's biggest one-day slide since late 2011.
The index has tumbled 11 percent since mid-September as doubts about the strength of the global economy mount.
After Wednesday's slump, all major European stock indexes are in negative territory for the year, with Germany's DAX among the worst hit, down 10.3 percent in 2014 and on track to record its worst annual performance since 2011.
The acceleration in selling was reflected in Europe's 'fear gauge', the Euro STOXX 50 Volatility Index, which surged to 28.9 on Wednesday, its highest level since mid-2012.
Shares in oil majors and oil services companies were hammered as Brent crude fell close to a four-year low around $84 a barrel. Total fell 4.5 percent, Repsol lost 4 percent and Statoil dropped 2.9 percent.
Norwegian seismic surveyor Petroleum Geo-Services shares tumbled 3.9 percent after the company cut its 2014 earnings forecast again, citing the fall in oil prices and worsening demand from oil companies.
The STOXX Europe 600 energy sector index is in bear market territory, down more than 20 percent since late June.
Pharmaceuticals stocks also featured among the top losers on Wednesday, after U.S. group AbbVie Inc said it was having second thoughts about bidding for British peer Shire because of changing U.S. tax regulations. Shire's shares plunged 22 percent.
Despite the correction, a number of fund managers see buying opportunities in European equities. They cite attractive relative valuation, the European Central Bank's recent measures to stave off deflation and support the economy and a slide in the euro currency, which should boost corporate earnings.
"A new recession in Europe has now been priced in, and the correction in stocks is getting close to an end. We now see good entry points, not exit points," said Romain Boscher, global head of equities management at Amundi, which has 821 billion euros ($1.04 trillion) under management.
"Even with no economic growth in Europe, there are plenty of positive factors supporting equities: very low refinancing costs for companies, a sliding euro which will boost margins, and very attractive dividend yields compared with what investors get in the fixed income space."
Europe bourses in 2014: link.reuters.com/pap87v
Asset performance in 2014: link.reuters.com/gap87v
Today's European research round-up
Additional reporting by Vikram Subhedar in London; Editing by Catherine Evans