55 points or 2.08 percent to close at 2,382.16, while the more liquid Milanka index went down 1.53 percent to 3,055.86, shedding 47.43 points.
Turnover was around 180 million rupees.
Brokers said the selling was by retail investors as well as margin traders seeking to reduce their exposure.
"The market is down with news of the bomb attack coming out," said R.
Muralidaran, Managing Director of Bartleet Mallory Stock Brokers.
Most shares fell although a few bucked the trend.
The indices were dragged down by market favourite Dialog Telekom, which fell 25 cents to 18.
25 with 286,700 shares traded, and Sri Lanka Telecom, which was down 75 cents to 29.75 with 169,400 shares changing hands.
"Investors are adopting a cautious approach because of the bombs," said Arjuna Dassanayake of DFCC Stockbrokers.
He said investors were aware they might have weather some rough times ahead.
The Tiger attacks in Buttala in the south east killed at least 28 people.
The Tigers first attacked a bus packed with early morning commuters, blowing it up with a roadside bomb and then shooting survivors.
At least 26 people were killed and more than 60 wounded in that attack.
In another attack, in a remote village in the area, apparently by the same group of Tigers later in the morning, four farmers were shot and hacked of whom two died.
The defence authorities previously said all four were killed.
And three soldiers were wounded by another roadside bomb which targeted an army vehicle.
Among the few gainers in the stock market was Beruwela Walk Inn, which shot up 9.75 rupees to 59.50, although most other hotel shares fell.
All plantations stocks were down as were most of the diversified companies except Carson Cumberbatch.
In forex markets the rupee gained against the greenback to 108.00/05 from yesterday's close of 108.
15/20 in late afternoon trading, dealers said.
Meanwhile at a treasury bill auction, rates fell across tenors.
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