Sensex ended the session at 81,757.73, down 501.51 points or 0.61 per cent against last day's closing of 82,259.24. The 30-share index started the session in the negative territory at 82,193.62 and dragged down further to hit an intra-day low at 81,608.13 amid selling pressure in banking heavyweights like Axis Bank and HDFC Bank, and Kotak Bank.