Mixed global cues before the US Federal Reserve starts its two-day meeting, also affected domestic sentiments.
After a better start at 20,732.44, the Sensex inched up to 20,784.03 on value-buying and positive cues from Asia.
However, it struggled at the day's peak and wiped off all gains to enter the negative terrain in the second half.
The 30-share Sensex closed down 47.38 points, or 0.23 percent, at 20,612.14 as 14 stocks declined while 16 rose.
Rate-sensitive banking, realty and auto shares were worst hit while defensive bets like healthcare and FMCG scrips saw buying. HDFC Bank, HDFC, NTPC and Coal India saw sharp losses while Bharti, Sun Pharma, Cipla and ITC rose.
The Sensex has now lost over 714 points in six sessions.
The 50-share NSE index Nifty fell by 15.65 points, or 0.25 percent, to 6,139.05, after touching the day's high of 6,190.55. SX40 index of MCX-SX fell 20.64 points to 12,264.64.
"Higher than expected inflation has fuelled expectations that RBI may hike rates in its meeting tomorrow. Similarly, outcome of Fed's meeting will have impact on Indian equity markets on Thursday," said Milan Bavishi, Head Research, Inventure Growth and Securities.
Expectations of a 0.50 percent hike by RBI on Wednesday have also built up after wholesale price index based inflation data came out higher-than-expected and price rise in retail level soared to over 11 percent, said analysts.
The initial gains today were also lost on weak opening in European stock markets, they added.
The BSE Banking sector index suffered the most by losing 1.28 percent, followed by PSU thematic index (0.83 percent), Power index (0.73 percent) and Realty index (0.66 percent).
Oil & gas index was also a loser dragged down by RIL.
Bucking the weak trend, the stocks of consumer durable, healthcare, FMCG, Teck, IT and capital goods sector gained and saved the market from any major fall, said dealers.


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