Raila Odinga said the result announced on Monday, declaring his rival William Ruto as president-elect, should be annulled. The dispute looks set to go to court.  NAIROBI, Kenya — Speaking for the first time since he lost Kenya’s hard-fought presidential election, the opposition leader Raila Odinga rejected the result announced on Monday and vowed to pursue “all legal options,” apparently signaling that the dispute that has gripped the East African country is likely to be decided at its Supreme Court. The narrow victory pronounced by the electoral commission in favor of his rival, William Ruto — 50.5 percent of votes against 48.9 percent for Mr. Odinga — was “a travesty and a blatant disregard of the constitution and the laws of Kenya,” Mr. Odinga told supporters at a conference center in downtown Nairobi. The results are “null and void and must be quashed by a court of law,” he added. Kenya’s election was largely lauded, until yesterday, as one of its most peaceful and best-organized votes. But Mr. Odinga’s accusations plunged the country into a period of uncertainty likely to last weeks, if not longer. However, his decision to take his challenge to the courts, rather than the streets, was a relief to many Kenyans who feared the argument might spiral into violence. “We had gathered stones and tires to burn; we were ready to go,” said Alex Shisia, a 46-year-old bus driver and self-described “Odinga die-hard” who had come to hear his leader speak on Tuesday. “But we are walking away and going home. ” A string of flattering newspaper headlines on Tuesday greeted Mr. Ruto, who is currently vice president. He moved quickly to cement his status as president-elect, issuing a call for national unity on Monday and taking congratulatory phone calls from the leaders of other African nations. He assured rivals there was “no room for vengeance” after a fiercely-fought campaign, and offered an olive branch to supporters of Mr. Odinga, who at 77 is making his fifth bid for the presidency, having lost his first four attempts. In his speech, Mr. Odinga accused the electoral commission chairman, Wafula Chebukati, of behaving like a “dictator” to swing the vote in favor of Mr. Ruto. And Mr. Odinga heaped praise on four election commissioners who had stormed from the counting center on Monday, moments before the result was expected to be declared, saying Mr. Chebukati had ignored their concerns about the counting process.   Mr. Ruto, for his part, has dismissed their actions as a “sideshow,” along with any suggestion that they have tainted the legality of his election.   The four dissenting commissioners, appointed last year by Mr. Odinga’s political ally, President Uhuru Kenyatta, gave a news conference on Tuesday where they outlined their reasons for refusing to verify the tally.