"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 damning inability and unwillingness of the Sri Lankan State and security forces to conceptualise and enact measures to ensure the safety and security of all citizens without curtailing and violating fundamental rights."
In June, authorities evicted some 400 minority Tamils out of their low-budget hostels in Colombo and took them to the troubled northern and eastern regions where Tamils are concentrated in the majority Sinhalese nation.
The expulsion was internationally condemned as collective punishment of Tamils for the work of Tamil Tiger rebels. The CPA spokesman said Tuesday that a lawsuit his group brought against the eviction was being heard.
The FMM, meanwhile, joined several media organisations Tuesday to press for an end to what it described as a culture of impunity on the island and for investigations into the killings of 11 people working for the press since November 2005.
FMM spokesman Sunanda Deshapriya said apart from the 11 killed, another five have been abducted. Two of them have been freed, but the fate of the other three remained uncertain, he said.
"The biggest single issue facing the media is this culture of impunity," Deshapriya said.
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