Rouhani, 64, a cleric considered a moderate pragmatist, replaced outgoing two-term President Mahmoud Ahmadinejad, who was barred from seeking a third term. Rouhani was elected in June in a surprise landslide victory.

Rouhani's formal inauguration and swearing-in was scheduled for Sunday, but Saturday marked the new president's ascension to office.

Iran's supreme leader, Ayatollah Ali Khamenei, formally endorsed Rouhani as president during a nationally televised ceremony attended by much of Iran's inner circle. Khamenei has the last say on matters of state in Iran's theocratic system.

Noticeably absent Saturday was former President Mohammad Khatami, considered the leader of Iran's so-called reformist bloc, which is deeply at odds with the conservative hard-liners who dominate the government. Khatami, viewed with extreme distrust by hard-liners, endorsed Rouhani's candidacy.

The ex-president's absence Saturday is an indication of the difficult balancing task facing Rouhani, who galvanized reformist support in his electoral victory but must avoid alienating powerful conservative blocs.

The supreme leader kissed Rouhani on the cheek, embraced him and patted him on the shoulders, flashing a smile. The two are old friends, but there have been reports of tension over Rouhani's perceived drift toward the reformist camp.

Rouhani is a longtime Islamic Republic insider and former parliament member and national nuclear negotiator. But he faces some major challenges, most notably: keeping his promise to turn around the nation's free-falling economy. He has also vowed to improve Iran's relations with other nations and work for the release of political prisoners.

In his speech, Rouhani warned that it could take some time to solve Iran's many problems, especially to reverse its economic tailspin. The nation suffers from a high unemployment rate and galloping inflation, problems exacerbated in part by U.S.-backed international sanctions tied to the nation's controversial nuclear program.

"There's a long way to our destination, and we just started our journey," Rouhani said. "Let's have peace and love and friendship, instead of enmity and antagonism."

In keeping with a campaign pledge, the new president vowed to work to remove the "brutal" international sanctions that target Iran's economy and hamper its access to the international banking sector and its ability to sell oil, the nation's primary export. But there was no mention during Saturday's ceremony of Iran's nuclear program, the underlying cause of the sanctions.

Iran says its nuclear efforts are solely for peaceful purposes. The United States and other Western powers believe that Tehran is trying to develop nuclear weapons.

The new president again emphasized his moderate stance, deliberately distancing himself from Ahmadinejad's provocative style.