UN chief Ban Ki-moon announced he would visit Sri Lanka as relief agencies and governments called for urgent humanitarian aid after Colombo said it had defeated the Tamil Tiger rebels.
Ban, who said he was relieved by the conclusion of military operations "but deeply troubled by the loss of civilian lives," will make a two-day visit on Friday and Saturday.
He said he wanted to see swift progress on immediate humanitarian aid, reconstruction, and sustainable political dialogue.
"Progress on all three fronts must be in parallel and it must begin now," he said, adding that any serious allegations of war crimes "should be properly investigated."
Meanwhile, a protest by Tamils outside Britain's parliament turned violent, leaving three police officers and five protesters in hospital while 10 protesters arrested.
The clashes broke out when police moved to disperse 2,500 Tamils who staged a sit-down outside parliament, with Scotland Yard saying another 21 officers received minor injuries.
Sivendran Madajrajah, a 22-year-old student, complained of police aggression, claiming people "were being hit, kicked and stepped on by police."
He added: "We feel really angry... how the British government is treating us when we are asking them for help."
Sri Lanka President Mahinda Rajapakse on Tuesday announced the final defeat of the Tamil Tigers, even as rebels insisted their leader was still alive and vowed to fight on for a Tamil homeland.
"The writ of the state now runs across every inch of our territory," Rajapakse said in a nationally televised address to Sri Lanka's parliament. "We have completely defeated terrorism."
As the UN Human Rights Council said it would hold a special session May 25 to examine the human rights situation in Sri Lanka, governments and aid agencies called on Colombo to step up relief and national reconciliation efforts.
British Prime Minister Gordon Brown said he had urged Rajapakse to ensure thousands of people displaced by the army's victory over the rebels got humanitarian aid and stressed the need to bring the Tamil minorityTamil minority into the political mainstream.
"I have offered humanitarian aid but asked (Rajapakse) to ensure that displaced persons are given the humanitarian aid that they need urgently," Brown said.
"This is potentially a humanitarian problem of very high proportions, (there are) large numbers of displaced persons," he said, adding that he had told Rajapakse "that he must begin a political process for involving the Tamil people and other communities in his country."
Japan's Prime Minister Taro Aso said it was "now important to help internally displaced people and their resettlement as well as to start showing improvement in the political process towards peace-building."
Beijing's foreign ministry spokesman said China urged Sri Lanka to move forward with reconciliation, while Russia welcomed the announcement of the rebels' defeat, saying it supported Sri Lanka's "fight against terrorism."
London-based Amnesty International asked Colombo to allow "unimpeded humanitarian access" adding that traumatised civilians who had made it out of the conflict zone were being housed in camps "swollen beyond their capacities." Around 280,000 civilians are housed in relief camps.
On Monday, US State Department spokesman Ian Kelly welcomed an end to the war, saying the United States was "relieved that the immense loss of life and killing of innocent civilians appears to be over."
The International Committee of the Red Cross, the only neutral organisation that was working in the conflict zone, has said no humanitarian aid has reached civilians in need for over a week.
Tens of thousands of civilians fled the fighting in recent weeks, raising fears of an humanitarian crisis, although the Sri Lankan government has refused to allow aid agencies or journalists into the war zone.
More than 70,000 people have died in battles, suicide attacks, bombings and assassinations since the Tamil Tigers began their struggle in the 1970s for an independent homeland for the Tamil minority.