More than $2.5 trillion have been wiped off the value of world stocks this week on mounting concerns the global economy is heading toward another recession and Italy and Spain are being engulfed by the euro zone sovereign debt crisis.

The sum wiped off the MSCI All-Country World Index is almost equivalent to the size of the entire French economy.

The MSCI All-Country World Index is down 8.6 percent this week, on track for its biggest weekly percentage fall since November 2008.

The U.S. S&P 500 index .SPX alone has lost more than $840 billion from its market capitalization this week, while European equities measured by the MSCI Europe .MSCIEU have lost more than $817 billion.

"These are markets to be careful (in), not to try and be a hero," Royal Bank of Scotland Chief Executive Stephen Hestor told reporters after the bank announced its first-half results.

Credit Suisse on Friday cut its year-end target for the S&P 500 to 1,350 points from a previous forecast of 1,450. The new estimate was still 11 percent above Thursday's close of 1,200.07.

The Swiss bank also cut its forecast for U.S. companies' 2011 average earnings per share (EPS) to 12 percent growth from 14 percent and for firms in the euro zone to EPS growth of 7 percent from a previous estimate of 12 percent.

However, not all were bearish.

Ralf Groenemeyer of Silvia Quandt Research said authorities may act soon to arrest the slide.

"We expect governments and central banks to act over the weekend ... It should not be ruled out that some kind of 'quantitative easing' measures emerge," Groenemeyer said.

Royal Bank of Scotland Credit Suisse Quantitative easing Credit Suisse