NAIROBI, Kenya — Kenya is embarking on one of the most perilous weeks in its political history, preparing for an election re-run that the main opposition group plans to boycott in a blow to a country once heralded as a beacon of regional stability.

The tension building ahead of the planned vote on Thursday has alarmed world leaders who are appealing for calm in Kenya, a linchpin of East African economic development that has played a key role in the fight against the Al-Shabab extremist group in neighboring Somalia, where a massive truck bombing killed at least 358 people on Oct. 14.

Pope Francis, UN Secretary-General Antonio Guterres and Moussa Faki Mahamat, chair of the African Union Commission, have called for restraint in a nation that endured deadly ethnic-based violence after a disputed 2007 election as well as the killing of dozens of people, mostly shot by security forces, after an August vote.

President Uhuru Kenyatta was declared the winner of the recent election, but in a surprise ruling, the Supreme Court nullified the vote because of what it said were illegalities and irregularities and ordered a new one within 60 days.

While Kenyatta has held campaign rallies ahead of the new election, challenger Raila Odinga has said his concerns about the transparency of the electoral process have not been adequately addressed and wants his supporters to protest in the days ahead, raising the prospect of more clashes with police as well as the disruption of polling stations.

Odinga's group, the National Super Alliance, said its leaders on Monday met Western diplomats in Nairobi and reiterated their belief that Kenya's electoral commission is not ready "to conduct free, fair and credible elections."

In a statement, the opposition group said it won't participate in the vote on Thursday "because it doesn't serve the country's interest."

Adding to the crisis atmosphere, two senior officials of Kenya's electoral commission had cast doubt on the whether a free and fair election can be held this week, saying the country is divided, the electoral commission is split, and Odinga, the major opposition candidate, refuses to stand in the poll.

Electoral commissioner Roselyn Akombe resigned, saying the commission cannot deliver a credible election. She fled to the United States, saying she feared for her safety. Shortly before the Aug. 8 vote, Christopher Msando, an election official who was in charge of the electronic voting system and technology to prevent voter fraud, was murdered in an unsolved case that fueled theories about alleged attempts to tamper with the electoral process.— AP