Posting its biggest single-day fall in nearly seven weeks, the Sensex at the Bombay Stock Exchange plunged 324 points, or 1.2 percent, to close at 26,493, today, on heavy selling by investors. The Nifty at the National Stock Exchange slumped 109 points, or 1.4 percent, to end below the 8,000-mark, at 7,933. Other key stock markets in Asia and Europe also fell, on investor caution ahead of the US central bank's two-day policy meet starting today.