ROME, Dec. 7 (Xinhua) -- Italian Prime Minister Matteo Renzi formally resigned Wednesday evening but President Sergio Mattarella asked his cabinet stay in office for day-to-day administration before handing over to a transition government.

He formally handed in his resignation to Mattarella after the country's 2017 budget was approved in Senate.

Renzi's resignation came three days after he suffered a sounding defeat in a referendum on a cabinet-backed constitutional reform held on Sunday.

Renzi had already submitted his resignation on Monday, but was asked by the president to remain in office until the budget bill passed, which happened Wednesday afternoon.

The resignation now opened the way for the president to launch a round of talks with all party leaders in order to name a new prime minister, and form a transition government.

Renzi's cabinet would remain in office for handling the state's administrative affairs, while the president "will start consultations at 6 p.m.(1700 GMT) on Thursday," Secretary General to the Presidency of the Republic Ugo Zampetti stated. The first round of talks is expected to end by Saturday.

Earlier in the afternoon, Renzi had gathered the leadership of his center-left Democratic Party(PD), which is the largest in parliament, to outline its political strategy for the government crisis.

"Our party does not fear democracy, nor vote. As such, if the other political forces want early elections, they just have to say it clearly," Renzi said, referring to the other parties calling for early elections.

He added the PD would also be open to a national unity government including all major parties, with the aim of overseeing a new electoral law, and the international appointments awaiting Italy in 2017.

"Yet, all the other forces will have to take the same responsibility in this case. The PD had already paid the price of solitude," Renzi stressed.

Renzi, now 41, became Italy's youngest prime minister in February 2014, pledging to implement a wide political and economic reform agenda.

The constitutional reform would have streamlined Italy's law-making process by demoting the senate, and leaving to the lower house only the power to bring down cabinets and control national legislation.

According to the oppositions, which joined forces in the No camp, the reform would concentrate too much power in the executive branch.

The reform was rejected by over 59 percent of voters on Sunday, and Renzi, as previously vowed, announced he would take the full responsibility of the defeat and step down.

Among opposition forces, anti-establishment Five Star Movement (M5S) and anti-immigration Northern League were now those pressing hardest for early elections.

Yet, in order to take this path, the current electoral law would need to be changed, or at least amended. In fact, it was approved in 2015 already foreseeing a demoted senate, as proposed in the rejected constitutional reform law, and does not provide complete rules for the upper house.

Furthermore, Italy's Highest Court will rule on the law's legitimacy next month.

For these reasons, president Mattarella was said to be much against new elections "before an homogeneous electoral law for both houses" was approved.