Independent National Electoral Commission president, Amadou Salifou Kebe, told reporters that 91.59 percent of ballots were in favor of adopting the new constitution, while 8.41 percent were against.
Turnout was 61 percent, he added, saying that these were provisional figures.
The vote was originally planned for March 1 but was postponed until March 22 because of international criticism of its fairness.
The authorities went ahead with it after scrubbing some 2.5 million unverifiable names from the electoral rolls, following advice from the West Africa bloc ECOWAS.
The day of the vote was marred by violence, with scores of polling stations ransacked across the country and, according to the country’s political opposition, dozens killed.
The proposal to change the constitution was hugely controversial in the West African state, spurring mass demonstrations in which at last 32 people have been killed, according to an AFP tally.
Authorities have said only a few deaths occurred on polling day, and that the voting took place in peace.
In a sign of impatience with Conde’s government abroad, however, France, the United States, and the United Nations have all expressed their reservations about the vote on March 22.
France, the former colonial power, condemned election-related violence in Guinea this week and said the vote was not credible.
The United States also said last week that the situation in Guinea was worrying and that it shared international concerns about the fairness of the vote, and the lack of political dialogue surrounding it.