The PTA was "used" mostly during the civil war against the Liberation Tigers of Tamil Eelam (LTTE) rebels but with the end of the conflict in 2009 some aspects of the law remained in force, the Xinhua report said.
The police spokesman said that the main charges for the arrest of the two activists were that they were attempting to create instability among communities and were also allegedly promoting separatism.
Meanwhile, the US embassy in Colombo called on the Sri Lankan government to ensure that due legal processes were followed in the case of the human rights activists.
The embassy said that the US government was concerned over the arrest of the two activists. It urged the government to ensure the two have full access to a legal counsel.
The Free Media Movement (FMM), a leading media rights group in Sri Lanka, condemned in the strongest terms the arrest of the two activists and called for their immediate release.
The FMM raised concerns over the arrests at a time when human rights issues in Sri Lanka were being discussed at the UN Human Rights Council currently meeting in Geneva.
UN High Commissioner for Human Rights, Navi Pillay, had in a report submitted to the UN Human Rights Council, raised concerns about the human rights situation in Sri Lanka.
The US has also raised concerns and submitted a draft resolution to the UN Human Rights Council which will be taken up for a vote next week.