Stock markets in the United States have fallen sharply - following earlier declines across Europe and Asia.
On Wall Street, the Dow Jones Industrial Average finished down 321 points lower - 2.4 percent -15,987.26, the S&P 500 was down 2.2 percent and the Nasdaq Composite index dropped 2.7 percent.
Phil Orlando, chief equity strategist at Federated Investors in New York, said: "Investors are scared to death."
In the London market, BHP Billiton shares dropped 6.5 percent. Among the other mining stocks, Anglo American fell 11.5 percent and Glencore slid 7 percent.
After a brief recovery on Thursday, oil prices fell again to hit fresh 12-year lows. The price of Brent crude dropped to $29.01 a barrel at one point, before nudging back to $29.16 while US crude futures reached a low of $29.39 before edging up to $29.66.
The sell-off on the UK market followed another big fall in Chinese shares, with the Shanghai Composite dropping 3.5 percent on Friday.
Markets across Europe were lower, with Germany's Dax index down 2.5 percent while in France the Cac 40 also dropped 2.4 percent.
In London, shares in telecoms group BT fell 0.8 percent to 463.3p, despite the Competition and Markets Authority giving final clearance to BT's takeover of mobile phone firm EE.
In the FTSE 250, shares in Moneysupermarket fell 11 percent after investors were rattled by a larger-than-expected fall in sales in the company's insurance unit during the fourth quarter. Revenues in the insurance business were down 10 percent compared with a year earlier.
On the currency markets, the pound hit fresh five and a half year lows against the dollar after the release of weak UK construction data.
Official figures showed construction output in November fell by 0.5 percent compared with the month before, and was down 1.1 percent from a year earlier.
Sterling had already weakened earlier this week after industrial production figures showed a fall in output.
The pound slid to $1.4296, a new five and a half year low. Against the euro, sterling fell by more than one euro cent to €1.3057.