Keeping its downward trend of morning trade, the BSE benchmark Sensex today was down by nearly 254 points to plunge below the 19,000 mark and was trading at 18,789 during noon.Similarly, the wide-based National Stock Exchange index Nifty also was down by nearly 71 points at 5690.The Japan's Nikkei fell by 4.67 per cent, while Hong Kong's Hang Seng shed 2.61 per cent. The US Dow Jones Industrial Average ended 0.84 per cent lower in yesterday's trade.