The BSE benchmark Sensex tumbled by 249 points in early trade on Thursday. Wiping out initial gains the Sensex, which opened over 191 points higher on Thursday to trade above the 20,000 mark, was down by 249 points, or 1.25 per cent, at 19,615 at 1030 hours.Similarly, the broad-based National Stock Exchange Nifty index lost 81.95 points, or 1.36 per cent, to trade at 5,906 after jumping to 6,076 in opening trade.