By Ed Cropley

PHNOM PENH (Reuters) - In a ceremony full of pageantry and hope, King Norodom Sihamoni ascended the Cambodian throne on Friday, ushering in a new royal era for a country still trying to draw a line under its bloody, genocidal past.

In accordance with Buddhist tradition, the 51-year-old Sihamoni, a previously unknown ballet aficionado, took triple oaths of dedication to Cambodia's 13 million people in a spectacular ceremony in the capital's gilded royal throne hall.

"As from this happy and solemn day, I shall devote my body and soul to the service of the people and the nation, pursuing the exceptional work accomplished by my august father, grandfather and great-grandfather," Sihamoni said.

His hastily arranged coronation came after the shock abdication of his ailing father, Norodom Sihanouk, this month.

State television broadcast blanket live coverage of the speeches, blessings and pageantry -- the end of an era for the war-scarred Southeast Asian nation, as Sihanouk, one of the world's most enduring Cold War leaders, bows out of public life.

The 81-year-old now ex-king, who will still bear a lengthy royal title, was not at the investiture, which was attended by Prime Minister Hun Sen, members of the National Assembly and scores of foreign diplomats.

Many Cambodians hope the smooth transition to a new head of state marks a fresh start for a nation still recovering from the horrors of the Khmer Rouge genocide, four years of terror in the 1970s which claimed an estimated 1.7 million lives.

At Buddhist temples across the country, monks banged huge wooden drums on the stroke of 6 p.m. to usher in the new monarch.

However, diplomats say it is unlikely Sihanouk will set his son, who has never held political office, loose in Cambodia's fractious and often bloody political arena without considerable guidance.

"I think we will see the hand of Sihanouk on the tiller of the monarchy for some time to come -- certainly until Sihamoni has found his feet," said one Western diplomat.