Voters Saturday choose representatives for a 200-seat National Assembly as a temporary government that will then be charged with drafting a constitution, Voice of America reported.
Another round of elections is scheduled next year to officially replace the government of former dictator Moammar Gadhafi.
Ban issued a video message to Libyans, reaffirming the United Nations' support of Libya's move toward democracy.
"On Saturday, you will exercise a right that you have been denied for more than four decades, the right to vote, the right to elect a new National Congress -- this is a milestone in Libya's long march toward democracy," he said.
"This is, truly, a moment for national celebration. We all know the road to democracy is long and hard. We share your aspiration for a peaceful and prosperous Libya founded on the principles of justice, human rights, inclusiveness and accountability."