The Free Media Movement, a press rights group, and the Centre for Policy Alternatives (CPA), a private think-tank, said they were considering legal action against the authorities for the weekend mass arrests.
"The CPA will seek court intervention on behalf of the people who were subjected to cruel and degrading treatment," a CPA spokesman said.
The government launched a major search operation in the capital over the weekend following last week's two bomb attacks that left 21 people dead and more than 40 wounded.
A senior government minister, Jeyaraj Fernandopulle, said nearly 2,200 people were arrested, but 1,800 of them had already been freed.
However, the Free Media Movement said Tamil civilians -- with all necessary identification and documentation -- were still in detention.
"The manner in which the arrests are being conducted and that they target a specific ethnic community are extremely worrying," the FMM said in a statement.
It said the arrests pointed to "the damnin