Sensex ended the session at 81,159.68, down 555.95 points or 0.68 per cent. The 30-share index started the session under pressure at 81,574.31 against the last session's closing of 81,715.63 amid overall selling. The index dragged further to hit an intraday low at 81,092.89. Nifty closed at 24,890.85, down 166.05 or 0.66 per cent.