The stock markets tumbled today. After a positive opening, the Sensex at the Bombay Stock Exchange retreated sharply, plunging a mammoth 661 points, to close at 27,188, on heavy, across-the-board selling pressure. The Nifty at the National Stock Exchange slumped 197 points, to 8,236. And at the inter bank forex market, the rupee depreciated 12 paise, to 63.82 against the dollar, on dollar demand from importers, and sinking domestic equities.