The Indian equity benchmarks erased early gains on Thursday ahead of monthly expiry of May futures and option contracts dragged by losses in Reliance Industries, ITC, Larsen & Toubro, Bajaj Finance, Asian Paints and Hindustan Unilever. The Sensex fell as much as 678 points from the day's highest level and Nifty 50 index broke below its important psychological level of 16,000.