UN Human Rights Commissioner Navi Pillay has sharply criticised the government of Sri Lanka, at the end of a week-long visit.
In a statement, Ms Pillay said she feared the country was becoming increasingly authoritarian.
Since the civil war ended four years ago, democracy had been undermined and the rule of law eroded, she added.
Ms Pillay is the most senior UN official to visit the north since Secretary General Ban Ki-moon in 2009.
She arrived in Sri Lanka last Sunday for a fact-finding mission after the government promised her access to former war zones.
She told reporters then that her job was to ensure that the government was conforming to human rights standards agreed by all nations.
The BBC's Imogen Foulkes in Geneva says the statement issued by Ms Pillay's office is astonishingly blunt, and parts of it will not go down well with the Sri Lankan government, which had not been especially keen on her visit in the first place.
Ms Pillay said that she was allowed to go wherever she wanted but that Sri Lankans who came to meet her were harassed and intimidated by security forces.
"This type of surveillance and harassment appears to be getting worse in Sri Lanka, which is a country where critical voices are quite often attacked or even permanently silenced," the statement said.
"Utterly unacceptable at any time, it is particularly extraordinary for such treatment to be meted out during a visit by a UN high commissioner for human rights."
Ms Pillay added that she would be reporting such incidents to the Human Rights Council.
In addition, she said she was concerned at recent attacks on religious minorities and at what she felt were government attempts to downplay them.
When she visited the northern city of Jaffna she met Tamil families who complained to her about missing relatives, military land grabs and life without basic facilities.
After Ms Pillay met the TNA, Sri Lanka's biggest Tamil party, one MP, MA Sumanthiran, told BBC Sinhala's Azzam Ameen they had raised concerns that "people who had met her in the north and east are now being harassed by military intelligence".
One Catholic priest told BBC Tamil that he had been questioned by unidentified men, which he felt was tantamount to "intimidation".
Ms Pillay told the BBC that she had raised the alleged harassment with the government, and said that she would "keep the focus" on what happened to the people she had met.
The government has vehemently rejected accusations that it had intimidated people or scaled down its military presence before she arrived, saying officials allowed her "to meet any one she wanted to" during her visit and that they had "nothing to hide".
Sri Lanka's army defeated separatist Tamil rebels after a brutal 26-year war in 2009. It is the final phase of that war which has come under scrutiny as well as the government's rights record since then.
Ms Pillay's visit comes after a second UN resolution in March urged Colombo to properly investigate killings and disappearances during the war, especially in its final stages.
The government has criticised the UN over the resolutions and insisted it did not massacre civilians.
Canada has called for a boycott of a Commonwealth summit scheduled to take place in the Sri Lankan capital Colombo in November.
The entire conflict left at least 100,000 people dead, but there are still no confirmed figures for tens of thousands of civilian deaths in the last months of battle: estimates range from 9,000 - 75,000.
One UN investigation said it was possible up to 40,000 people had been killed in that time. The government puts the figure at 9,000.