World stock markets tumbled on Friday and central banks in the U.S., Europe and elsewhere continued an unprecedented infusion of cash into the financial system, as concern spread about the state of the U.S. credit market and the complicated array of investments it supports.

On Wall Street, the Dow Jones Industial Average was down nearly 200 points after the first hour and a half of trading on a day that was expected to add to Thursday's 387-point drop. Though still up for the year, Thursday's decline was the second-worst of the year and knocked nearly three percent from the index's value. Other U.S. indexes were down as well...

It was much the same overseas, as benchmark indexes across Asia and Europe shed upwards of three percent on Friday. Japan's Nikkei 225 lost 406 points, a decline of 2.37 percent, while the Hang Seng Index in Hong Kong lost 2.88 percent.

Europe followed suit: near the end of the trading day, London's FTSE 100 had fallen 184 points, a decline of nearly three percent; France's CAC 40 was down around three percent and Germany's Dax 30 had shed nearly 1.4 percent.

To calm the markets and meet a surge in demand for cash, central banks on Friday continued pumping money into the financial system -- adding to the more than $150 billion that authorities in the U.S. and Europe released on Thursday...

"What we have at the moment is just an all-around sense of panic," Marc Ostwald, a bond analyst at Insinger de Beaufort in London, told the Associated Press. "Quite clearly there's a lot of deep-seated fear out there and it's going to take a while to resolve this."

The sell-off overseas represents the expansion of a problem first glimpsed through rising default rates for U.S. home mortgages -- particularly among riskier "subprime" loans to less creditworthy borrowers. It has evolved into a global credit crunch, with borrowing costs for corporations and global dealmakers on the rise, and investors urging policymakers to help.