After warning throughout last year’s revolt of a descent into Somalia-like chaos of tribal infighting , Mr. Vandewalle said that Libyans so far had proved him wrong. “Who would have predicted a year ago that there would even be elections?” he asked.

Still, even amid the jubilation — joined by the young militia men in artillery-mounted pickup trucks who are both the source of Libya’s tenuous security and its scourge — there were many reminders that the vote was just the beginning of the struggle to unify the country and build a new democracy.

The vote will select a 200-member congress that was initially expected to draft a constitution while it governed the country for 18 months. But the interim Transitional National Council stripped the congress of that authority just two days before the vote in an attempt to placate easterners who protested that the congress would be stacked in favor of the more populous West around Tripoli. (One hundred members will be elected from the west, 60 from the east, and 40 from the desert south.)

The council instead decreed a new election to choose a smaller panel to draft the constitution that would be composed of equal numbers from each region.

The last-minute change all but guarantees that the first move by the new congress, whose members campaigned to be part of a constitutional assembly, will be to challenge the council’s new plan and with it the legitimacy of the rest of its road map for transition. The council has pledged to dissolve itself at the seating of the new congress.