The BSE benchmark Sensex tumbled by 191 points in opening trade today. The 30-share index, which has lost 1,198 points over the past seven sessions, fell further by 191 points, or 1.16 per cent, to 16,180 in the first five minutes of trade today.In a similar fashion, the wide-based National Stock Exchange Nifty index declined by 58 points, or 1.18 per cent, to 4,847.Meanwhile, in the Asia region, Japan's Nikkei Index fell by 0.21 per cent, while Hong Kong's Hang Seng Index was down by 1.86 per cent in early trade today.