Sensex tumbled as much as 650 points to its day's low of 81,608.13 on Friday, marking a nearly 0.80% decline from yesterday's close. FIIs turning net sellers in July, a 6% drop in Axis Bank shares over missed financial earnings data and Citi's downgrade of Indian equities from 'overweight' to 'neutral' caused the dip. Rising oil prices also contributed.