More than ten thousand Tamils on Monday protested in front of the United Nations in Geneva, accusing the world body of complicity in a Sri Lankan "genocide".
The demonstration in front of the UN's European headquarters was part of a worldwide series of protests by the Sri Lankan minority. Organisers had pegged the turnout at 30,000.
An association for Tamils in Switzerland, the Tamil Forum, released a letter to UN Secretary-General Ban Ki-moon condemning what it called the evacuation of civilians in Sri Lanka and their placing in "army detention camps".
On Monday, the European Union appealed to Sri Lankan authorities and the Liberation Tigers of Tamil Eelam (LTTE) rebel group to agree to an immediate cease-fire to allow urgent humanitarian aid into a northern "safe zone" where civilians are supposed to be shielded from the fighting.
On Friday, Navi Pillay, the UN High Commissioner for Human Rights, accused both sides of human rights violations.
The UN believes 150,000 to 180,000 civilians remain trapped in the territory still under rebel control.
LTTE rebels have fought since 1983 for an independent state for Tamils, which suffered decades of marginalisation at the hands of governments dominated by the Sinhalese majority.
More than 70,000 people have been killed in the fighting.