UN Human Rights Commissioner Navi Pillay has visited the former north Sri Lankan Tamil Tiger stronghold of Jaffna to meet relatives of people who disappeared at the end of the war.
Tamil families complained to her about missing relatives, military land grabs and life without basic facilities.
Ms Pillay is the most senior UN official to visit the north since Secretary General Ban Ki-moon in 2009.
She told survivors that she would raise their complaints with the government.
She is due to meet senior ministers including President Mahinda Rajapaksa later this week.
Former conflict zones in the north have, until recently, been off limits to foreign journalists, aid workers and UN staff.
About 300 people demonstrated in Jaffna to complain that the UN was not doing enough to investigate enforced disappearances and land-grabs.
One of the protesters, Ananthi Sasitharan, is the wife of Velayutham Sasitharan, a top Tamil rebel leader.
Tamil protesters in Jaffna displayed photos of disappeared people outside the main library where Ms Pillay was holding a meeting
"I am confident that he is alive. He is somewhere in a secret detention centre," she told the Reuters news agency.
At the same time another demonstration was held by pro-government Buddhist monks in Colombo, calling on Ms Pillay to leave the country and stop criticising its rights record.
She arrived in Sri Lanka on Sunday for a fact-finding mission after the government dropped its public hostility to her and promised access to former war zones.
Ms Pillay has told reporters that her job is to ensure that the government is conforming to human rights standards agreed by all nations.
Tamil Tiger rebels fought a 26-year war for a separate state in the north and east before they were defeated in 2009.
Rights activists say that some of those who disappeared were fighting for the Tamil Tigers, some fought for the government and some were civilians.
Ms Pilay’s seven-day visit comes after a second US-sponsored UN resolution in March urged Colombo to carry out credible investigations into killings and disappearances during the war, especially in its final stages.
Human rights activists in Jaffna say more than 700 people disappeared between 2006 and 2009.
Ms Pillay on Tuesday also visited the former Tamil Tiger stronghold of Killinochi and the eastern town of Trincomalee.