The Bombay Stock Exchange benchmark Sensex on Friday tumbled over 451 points in early trade on hectic selling by funds. The Sensex, which had witnessed two big losses this month, tumbled 451.63 points to 16,403.30 points in the first ten minutes of trading. Similarly, the wide-based National Stock Exchange index Nifty dropped by 140.50 points to 4865.05 points.Brokers said the selling focus was more on banking and realty stocks after Dubai's debt problems revived concerns about the global financial system and rattled markets across Europe and Asia.