The Sensex at the Bombay Stock Exchange, which had opened weak, later slumped as much as 421 points, or 2.1 per cent, to fall below the 20,000 level, at 19,889, a short while ago, as selling pressure intensified. Earlier, the Sensex had opened 64 points lower, at 20,246 in morning deals, on profit-booking and a mixed trend in the Asian region.Other Asian bourses were mostly lower, today, with key benchmark indices in Japan, China, Singapore and Hong Kong losing between 0.1 per cent and 1.8 per cent as an interest rate hike in South Korea added to speculation that China will also tighten monetary policy to cool inflation.