Sensex today sharply declined by 344 points on heavy selling by funds on expiry of current month contracts in the derivatives segment amid weak global cues. The 30-share index plunged by 344 points to close at 16,855 points, recording the biggest fall after 3rd November.The wide-based National Stock Exchange index Nifty fell by 103 points to 5,006 points, after touching the day's low of 4,986 points. Selling pressure was more confined to banking, oil and gas, consumer durables, realty and IT stocks.