Demonstrations have continued for a second day outside the UN's Colombo offices with protesters angry over its probe into alleged war crimes.
UN staff in Colombo have been asked to work from home. Only a few key people are at the office in the city centre.
The BBC's Charles Haviland in Colombo says that the UN operation in Sri Lanka has been significantly disrupted.
Housing Minister Wimal Weerawansa again led the protests on Wednesday. He argues that the probe is unnecessary.
'Strike to death'
"We're fighting for the withdrawal of Ban Ki-Moon's advisory panel," Mr Weerawansa told the BBC.
"Today they have a chance to respond to our request. If not, from tomorrow one of our representatives, a parliamentarian, will start a hunger strike to death."
The government denies its troops committed war crimes but some foreign governments and international organisations say there is evidence to the contrary.
On Wednesday the US said that it supported Mr Ban's advisory panel.
State Department spokesman Mark Toner said that while people had the "right to free expression" it was also necessary to have a "robust accountability process" to help reconciliation in the island after decades of war.
On Tuesday the UN said it "strongly" objected to the protest at its compound, which was hindering its "vital work".
The government has pledged to ensure UN staff are able to enter and leave their workplace, but our correspondent says it is tacitly approving of Mr Weerawansa's demonstration.
It has now ordered the police away from the demonstration and said that it "understands" that the protest will continue.
The opposition say that they fear the events will bring shame and disrepute on Sri Lanka.
The government has refused to grant visas to the UN advisory panel's three members, saying the investigation violates its sovereignty.
There have been consistent allegations that both the army - and Tamil Tigers rebels who troops routed last year - committed crimes at the end of the war.
About 7,000 civilians died in the last five months of the war, according to the UN.
It says the panel, announced last month, will report back within four months and will advise on how to deal with alleged perpetrators of abuses.
The protesters, many of them Buddhist monks, say their action will continue until the UN disbands the panel. The UN insists there are no plans to disband it.