We "will safeguard the constitution, (state) institutions, sovereignty, independence and the project of the building of the Lebanese state," Hariri told reporters at Baabda palace after President Michel Suleiman designated him Lebanon's premier and tasked him with forming a new cabinet.
He vowed to form a "homogeneous" and "consensus" government that is able to remain united and "make achievements away from obstruction" in cooperation with the president.
Hariri also promised to keep strife and "foreign and economic" dangers away from Lebanon.
Suleiman on Saturday designated Hariri Lebanon's premier after the majority of parliamentarians nominated the politician to the post.
"According to the constitution and after the president consulted with the speaker of parliament and parliamentarians, he (Suleiman) summoned Saad Hariri and tasked him with forming a new government," a presidential statement said.
The decree issued by Suleiman came after Hariri received 86 votes of approval by lawmakers following two days of consultations. Hariri got the approval of 71 MPs from his own majority alliance, plus Parliament Speaker Nabih Berri and his bloc of 12 legislators and two Armenian MPs.
This would be Hariri's first designation to the premiership following the June 7 legislative elections in which the March 14 coalition won 71 out of the 128 seats in parliament.
The 39-year-old is to succeed outgoing Prime Minister Fouad Siniora, a member of Hariri's al-Mustaqbal movement.
From Baabda palace, Hariri headed to downtown Beirut and prayed at his father's graveside along with Siniora and other members of his bloc.
Hariri now faces the difficult task of negotiating with other political factions to form a government. The process can take days or even weeks.
As news broke up about his nomination, celebratory gunfire shook Beirut and its suburbs. However, Hariri urged citizens in a statement not to fire in the air.
Around 16 people were injured from celebratory gunfire on Thursday, when parliament reelected MP Nabih Berri as the speaker .
