The BSE Sensex tumbled by over 262 points in opening trade today amid sustained selling pressure influenced by a weakening trend in Asian markets on worries over the euro zone debt crisis.The 30-share index, which lost 244 points in the previous session on Friday, plummeted further by 262 points, to around 16,192 in opening trade today. In a similar fashion, the wide-based National Stock Exchange Nifty index declined by 77 points, to around 4,866, led by stocks of metal and banking firms.