The Bombay Stock Exchange benchmark Sensex fell over 155 points to trade below the 18,000 level in opening trade today on sustained selling by funds in line with a weak global trend. The 30-share index, which had lost about 149 points in the previous session, fell by 0.86 per cent to 17,914 points, with all the sectoral indices led by metal, IT and realty trading in negative zone with losses up to 1.36 per cent. The wide-based National Stock Exchange index Nifty breached a psychological 5,400 level to trade lower by over 48 points to 5,372 points.Traders said the selling pressure was sparked by a plunge in world markets on investor worries over fears of a global economic slowdown, led by the United States and China. Besides, weak opening in Asian stock markets, which were down up to 2.02 per cent in the morning trade today, also dampened the sentiments, they added.