The main index fell 0.4 percent, or 23.33 points, to 5,745.55, from its highest close since Feb. 18.
Turnover was 353.4 million rupees, the lowest since March 18 and well below this year's daily average of 1 billion rupees.
Foreign investors were net buyers of 101.6 million rupees worth of shares, extending the year-to-date net foreign inflow to 4.54 billion rupees ($35.79 million) worth of shares.
'Trading was very dull and there was no buying support at all,' said a stockbroker on condition of anonymity.
Stockbrokers said the market was concerned over a new resolution in which the United Nations urged Sri Lanka to carry out credible investigations into killings and disappearances during its nearly 30-year civil war.
Yields in T-bills rose for the fourth straight week at Monday's auction even though the central bank had kept its key policy rates unchanged for a third month in a row.,
The rupee edged down to 126.73/75 to the dollar, sliding from Friday's close of 126.70/75, on slight importer demand for dollars, dealers said. ($1 = 127 rupees)
(Reporting by Shihar Aneez and Ranga Sirilal)
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