After opening higher, the index touched day's high of 18,801.10 but the BSE benchmark index started trimming gains as investors sold positions in auto stocks.
Meanwhile, Prime Minister Manmohan Singh met President Pranab Mukherjee, triggering fresh speculation of a Cabinet reshuffle. The meeting lasted one hour, official sources said.
Brokers said investors, confused over the on-going political chaos, decided to book profits and play safe ahead of the expiry of derivatives contract on October 25.
RIL also gave up initial gains and closed 1.21 per cent down as selling pressure intensified.
Dealers said a positive opening in European indices also failed to enthuse the domestic market sentiment and the 30-share Sensex ended the session down 135.85 points, or 0.73 per cent, at 18,577.70, a level last seen on September 27.
As many as 23 stocks reported losses led by Mahindra & Mahindra (3.66 pc), Tata Motors (2.62 pc) and Tata Steel (2.45 pc). SBI, Infosys and ITC also ended lower.
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Brokers said sentiment dampened as investors also focussed on RIL reporting fourth straight quarterly drop in profit.
Auto scrips fell on lower-than-expected sales, while metal stocks were hit on fears of slowdown in China hitting demand.
In all, 11 out of 13 sectoral indices closed lower. The market breadth was negative as 1,712 stocks ended with losses while 1,179 shares closed with gains.
The falling trend was to some extent capped on firming trend in Asia and Europe after US retail sales beat estimates, improving outlook for Indian exporters, they added.
Likewise, the 50-share National Stock Exchange index Nifty fell by 39.25 points, or 0.69 per cent, to 5,648. (MORE)