"Two people were arrested at 10.30 last night under the provisions of the Prevention of Terrorism Act. They were acting so as to promote communal disharmony," police spokesman Ajith Rohana said.
The arrests came ahead a key meeting of the UN Human Rights Council in Geneva later this month that is expected to pass a US-sponsored resolution against Sri Lanka over its alleged rights abuses during the ethnic war with LTTE rebels.
They were arrested from the northern town of Kilinochchi.
The Kilinochchi's police chief said: "Ruki Fernando and Rev Praveen behaved in a suspicious manner as they visited some families of those who have lost their family members."
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The UN Human Rights Council resolution is the third in as many years on Sri Lanka's accountability and reconciliation with the Tamil minority after the end of the 37-year ethnic conflict.
Up to 40,000 Tamil civilians were killed at the end of the separatist war in 2009, rights groups and UN experts have said.