Kenyan opposition leader Raila Odinga arrives at the Supreme Court in Nairobi in September. He challenged the Aug. 8 presidential election, in which he was the losing candidate, as flawed. (Ben Curtis/AP)

Kenya's main opposition leader announced Tuesday that he was dropping out of a rerun of the August presidential election, casting doubts on the legitimacy of the voting process.

Kenya is a key U.S. partner on the continent and has the most dynamic economy in East Africa, but its rowdy elections are often plagued by violence, threatening the country's stability.

Even for Kenya's often lively politics, however, the latest developments mark a plunge into uncharted waters, with first an election outcome struck down and now a candidate walking away.

It was a legal challenge by opposition politician Raila Odinga that prompted the Supreme Court, citing irregularities, to nullify the results of the Aug. 8 election, won by incumbent Uhuru Kenyatta. But Odinga, who leads the National Super Alliance (NASA), says not enough has been done to address the problems. He has singled out the Independent Electoral and Boundaries Commission (IEBC) for particular criticism, deeming it "rotten."

"We have come to the conclusion that there is no intention from IEBC to make sure that the irregularities and illegalities witnessed before do not happen again," Odinga told reporters Tuesday as he announced his withdrawal from the new election planned for Oct. 26. "We believe that all will be best served by NASA vacating its presidential candidacy in the election."

Legal expert Dunstan Omari described the stalemate as a "constitutional crisis" and said the two main parties — Odinga's NASA and Kenyatta's Jubilee Party — would have to consult the courts for guidance.

He said the Supreme Court is most likely to rule that new candidates will have to be found in the next 90 days, though other experts said the election could go ahead with just one candidate, akin to a yes-or-no vote.

Odinga called for nationwide protests Wednesday.

Politics in Kenya have often been marred by violence, frequently along ethnic lines. At least 37 people died in the August vote, but in 2007, at least 1,400 people died in a wave of violence after Odinga lost to Mwai Kibaki, the incumbent.

For his part, Kenyatta said the election would go forward, according to the Nation newspaper. While Odinga had the right to pull out, the president said, "he should know that Kenyans have a right to choose their leaders."

In its ruling invalidating the August election, the high court cited a consistent pattern of irregularities in polling stations reporting results. It ordered a new vote within 60 days.

Kenyatta had said he would abide by the court's decision but repeatedly accused the justices of standing in the way of the people's will. He also said he would use his party's majority in parliament to impeach Odinga if the latter won.

In the run-up to the new election, the Jubilee Party attempted to overhaul electoral laws and the IEBC, but the opposition dismissed the efforts as insincere.

"The Jubilee administration's proposed amendments to election laws demonstrate that it has no intention of competition on a level playing field," Odinga said. "All indications are that the election scheduled for October 26 will be worse than the previous one."

Opposition lawmakers on Tuesday boycotted a session of the National Assembly dedicated to amending the election laws.

John Otieno, an Odinga supporter, said he was very disappointed that his candidate would not run. He said he had hoped that the ruling party and the electoral commission would heed demands for reforms.

"Raila was our only hope. Our future for now seems very uncertain," Otieno said. "We will wait to see what happens next, but for now, we will heed his call of countrywide protests starting tomorrow."

Odinga's supporters have been holding weekly demonstrations, in the capital as well as in their strongholds of Mombasa and the western city of Kisumu, to pressure the electoral commission.

The protests have been heating up, with police opening fire on a demonstration in Nairobi on Monday, injuring one man. Last week, the University of Nairobi was shut down indefinitely after clashes between students and police.

Supporters of Kenyatta, however, dismissed Odinga's withdrawal as political theater and said he was never ready for office.

"We want Kenyatta to be sworn in as early as tomorrow so that we can celebrate," said Francis Kinyajui. "This was a joke; it was a waste of time. These were just plans to cause divisions in the country."

Amid the political furor, gunmen attacked a university near Mombasa along the coast and killed two staff members.

The attackers, thought to be members of neighboring Somalia's al-Shabab Islamist group, opened fire on a vehicle carrying university staffers.

Schemm reported from Addis Ababa, Ethi­o­pia.