NAIROBI, Kenya — Several of Sudan’s biggest opposition parties announced Thursday that they were planning a full boycott of the national elections this month, casting a cloud of uncertainty over Sudan’s first multiparty vote in more than 20 years and complicating the landmark agreement that ended decades of civil war in the country.

The warning followed a surprise announcement the night before by the leading opposition candidate, Yasir Arman, that he was dropping out of the presidential race because it was “impossible” to hold an election in the conflict-racked region of Darfur and that the whole electoral process had been “rigged.”

However, he said that his party, the Sudan People’s Liberation Movement, which fought a long and bloody war against Sudan’s government, would continue to participate in the parliamentary and local elections across the country, except for Darfur.

On Thursday, more than half a dozen other important opposition parties held a long meeting in which they concluded that the elections would be so flawed that they did not want to participate at all. Still, the parties seemed to allow themselves some wiggle room, with opposition leaders saying that they needed to consult their members before making any final decisions.

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