The FTSE 100 index closed down 80.94 points, or 1.5 per cent, at 5,260.99 just above a support level of 5,259 given by analysts.
The blue-chip index is down 5.3 per cent for the week, on track for its worst weekly losses since early March 2009. It is down 2.8 per cent for the year-to-date, after having been up almost eight per cent at the start of April.
Banks were the sharpest fallers on the FTSE 100, as European Central Bank President Jean-Claude Trichet did little to quell concerns over the debt crisis that are coursing through the euro zone.
Royal Bank of Scotland, Barclays, HSBC, Standard Chartered and Lloyds Banking Group fell 3.7 to 6.5 per cent.
Supermarkets group WM Morrison lost 3.2 per cent after posting a sharp drop in underlying quarterly sales growth.
Fellow food retailers J Sainsbury and Tesco shed 2.3 and 1.7 per cent, respectively.
Crude and metal prices fell across the board as the uncertainty over the euro zone hit demand fears and forced investors to the traditional safe haven of the greenback.
The UK mining sector has lost more than nine per cent in the past week.
Integrated oils Royal Dutch Shell and BG Group fell 2.1 and 1.8 per cent, respectively.
But peer BP gained 0.4 per cent. Voting was underway in the British general election, with overnight opinion polls showing the Conservative lead firming, leaving it a close call as to whether they will win an overall parliamentary majority.