The Sensex at the Bombay Stock Exchange surged 326 points, or 1.7 percent – the biggest single day gain in two weeks – to close above the 20,000-level, at 20,031 on Monday, on all-round buying amid rising European markets.The Nifty of the National Stock Exchange jumped 100 points or 1.7 percent, to 6,083. Stock markets in Japan, China, Hong Kong, South Korea and Singapore ended mixed.