Latortue received the presidential decree naming him prime minister at a ceremony attended by diplomats, politicians, and the council of seven eminent Haitians who chose him, popularly known as the "council of wisemen."
During the ceremony, Alexandre spoke of "the common vision of national reconciliation" that he shared with the new prime minister.
He also denounced "the passion, the hatred, the vengeance and the corruption at the highest levels of the government" of deposed president Jean Bertrand Aristide.
"Your failure would also be ours, and that of the entire Haitian population. We are condemned to succeed," Alexandre said.
Latortue said he hoped to name a new government by Monday morning at the latest, with the new ministers taking office as early as Wednesday.
"This country cannot continue to wait, the void has lasted too long. We must get to work as soon as possible," he said.
Latortue, in efforts to pick a national unity government, met earlier on Wednesday with members of deposed president Jean Bertrand Aristide's Lavalas party, with representatives of opposition parties and of of Haitian business, industry and labour.
While they agreed on the need for national reconciliation, Latortue said that would not mean "impunity in cases where crimes or abuses have clearly been committed."
"This country cannot continue to wait, the void has lasted too long. We must get to work as soon as possible"
Haitian interim president
"I will personally see to it that every dollar given for development in the Republic of Haiti is spent in line with the purpose for which it was given," he said.
Latortue said he would travel to other cities to "reassure" people, especially in the northwestern town of Gonaives, his hometown where the anti-Aristide uprising began.