Party boss Renzi was set to be picked by President Giorgio Napolitano after a day of consultations with political leaders at the presidency including disgraced former premier and opposition leader Silvio Berlusconi.
The anti-establishment Five Star Movement protested what it called an undemocratic power grab by Renzi and boycotted the talks with Napolitano, saying Italians should be allowed to choose through elections.
Letta stepped down yesterday after the Democratic Party approved a motion calling for a new government proposed by the 39-year-old Renzi, an ambitious former Boy Scout who was elected to the party leadership in December.
Opinion polls show Renzi enjoys high popularity ratings, mainly because as someone with no experience in national government or parliament he is seen as a welcome breath of fresh air in Italy's discredited political system.
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But the polls also indicate that most Italians would have preferred early elections and are opposed to what critics defined as a "palace coup" engineered by Renzi following weeks of increasingly bitter feuding with Letta.
Italy's economy showed signs of emerging from a devastating recession, with a preliminary estimate yesterday showing it grew 0.1 percent in the last quarter of 2013 in the first positive result in two years.
Napolitano began his day of consultations today by meeting with the smallest parties in parliament and will work his way through to a final meeting with the Democratic Party scheduled for 1815 GMT.
If Renzi receives the nomination he will then have to hold his own consultations on forming a government in the coming days and analysts are predicting that the new cabinet could be installed by the middle of next week.