The group, the Civil Monitoring Committee, said they suspected pro-government elements had planted a bomb in a low-budget hostel occupied by members of the Tamil community as part of the pressure tactics.
"We have reasonable suspicion that the bombs were planted yesterday to put pressure on the residents there to withdraw their cases against the police," committee spokesman Mani Ganeshan said.
"We have seen similar things in the past and the government is responsible for this pressure," the spokesman said.
There was no immediate reaction from the government.
But the defence ministry Wednesday said that two Claymore mines and three hand grenades were found at the Ideal Guest Inn following a tip-off.
Seven men who had challenged the forced eviction of nearly 400 Tamil men, women and children from the capital Colombo in June were staying at the Ideal Guest House.
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"The seven petitioners are now under pressure to withdraw the case that is before the Supreme Court," Ganeshan said