(CNN) -- At their president's urging, Sri Lankans took to the streets Sunday to rail against a critical United Nations report that found credible allegations of war crimes and crimes against humanity committed by both sides during the final stages of the country's civil war.
Hundreds turned out for the peaceful demonstration in Colombo, answering a call from President Mahinda Rajapaksa. The protesters, who joined up with a larger May Day rally demonstrating for worker's rights, targeted the United Nations, some of them holding up signs tweaking the world body's Secretary-General Ban Ki-moon.
It was in response to a U.N. report released Monday, in which a three-member panel recommended that Sri Lanka immediately conduct an investigation into the alleged violations of international law and take other measures to "advance accountability."
Human rights groups have already alleged that both government troops and Tamil Tiger rebels violated humanitarian laws and that thousands of civilians were killed during the war, which ended in May 2009 after the government declared victory. The rebels had fought a 26-year bloody separatist war that left thousands dead and large numbers of others internally displaced, according to the United Nations.
The report concluded there were "credible allegations, which if proven, indicate that a wide range of serious violations of international humanitarian law and international humanitarian rights law was committed both by the government of Sri Lanka and the LTTE (rebels), some of which would amount to war crimes and crimes against humanity."
In the war's final stage, which lasted from September 2008 to May 2009, the Sri Lankan army advanced into an area of northern Sri Lanka known as the Vanni, where about 330,000 people were trapped by fighting.
The report said the government used "large-scale and widespread shelling" that left large numbers of civilians dead.
Some of the shelling happened in no-fire zones where the government had encouraged civilians to congregate, the report said. Government forces also shelled a U.N. hub, food distribution lines and fired near ships belonging to the International Committee of the Red Cross that were picking up the wounded and their relatives from beaches, the report said.
The government shelled hospitals on the front lines as well, some of them repeatedly, the report said.
"Most civilian casualties in the final phases of the war were caused by government shelling," the report said.
The U.N. secretary-general's spokesman said Ban "sincerely hopes that this advisory report will make a contribution to full accountability and justice so that the Sri Lankan government and people will be able to proceed towards national reconciliation and peace."
CNN's Aliza Kassim and journalist Iqbal Athas contributed to this report.