By Helen Murphy and Julia Symmes Cobb
BOGOTA (Reuters) - Colombia's President-elect Ivan Duque was sworn in to office on Tuesday, pledging to unite a divided nation behind his plan to toughen a peace accord with Marxist rebels and rekindle economic growth.
Right-wing Duque, who replaces Nobel Prize winner Juan Manuel Santos, faces significant challenges.
The economy remains weak, a new wave of drug trafficking gangs have moved into areas once controlled by the Revolutionary Armed Forces of Colombia (FARC) guerrillas, and nearly a million Venezuelan migrants have crossed into Colombia looking for food and work.
The 42-year-old lawyer and former senator for the Democratic Center party won a decisive victory against a leftist opponent in June's election, promising to make adjustments to the domestically-controversial peace accord with the FARC, cut corporate taxes and redouble security efforts in certain areas.
"I want to govern Colombia with unbreakable values and principles, overcoming left and right divisions," Duque said in an address before dignitaries, after receiving the presidential sash in Bogota's Plaza Bolivar.
"I want to govern Colombia with the spirit of building, never of destroying."
Ongoing peace negotiations with National Liberation Army (ELN) rebels, the country's last remaining insurgent group, will be evaluated over the next 30 days, Duque said, adding that any process must be "credible" and based on an end to guerrilla criminal activity over a specified timeframe.
He said he will also send an anti-corruption bill to Congress and launch measures to reactivate the sluggish economy.
Duque is a protege of hardline ex-President Alvaro Uribe, a harsh critic of the peace agreement, whose father was killed by rebels.
Uribe, facing allegations of witness tampering and bribery that he has denied, is seen by many as the power behind the relatively inexperienced Duque.
But Duque, a father of three who worked at the Inter-American Development Bank before Uribe asked him to take a Senate seat in 2014, has already shown independence in some cabinet picks and in a softening of his anti-accord rhetoric.
The 2016 peace deal brought an end to the FARC's part in more than five decades of war which killed some 260,000, and saw thousands of rebels demobilize in return for amnesty.
And though its leadership will be tried for war crimes, Duque is angry they will not serve jail time before taking up 10 guaranteed congressional seats.
Colombia's youngest president of the modern era has not specified the changes he would make to the agreement, but anything more than cosmetic will be tough to get through a Congress which has broadly backed the deal.
Even so, alterations may help satisfy some detractors in a nation polarized by the deal.
Duque can count on continued support from the United States, said Nikki Haley, U.S. ambassador to the United Nations, in Bogota.
"He very much sees the fact that the peace process, while it happened and was historic, wasn't perfect and so now it needs to be tweaked a bit," she told journalists.
The prematurely gray-haired Duque has also said he plans to cut taxes and raise revenue from an evasion crackdown. He wants to relax the so-called fiscal rule, which obliges the government to reduce the budget deficit.
"In economic matters, mistakes have been made that we must never again repeat," said Duque. "A tax policy motivated by the expansion of spending has led us to having asphyxiating burdens that affect savings, investment, job formalization and productivity."
With Colombia's debt rated BBB- by S&P and BBB by Fitch, Duque will have a hard time satisfying credit rating agencies unless he is able to replace revenue lost from weaker international oil prices.
(Reporting by Helen Murphy and Julia Symmes Cobb, Additional reporting by Nelson Bocanegra, Carlos Vargas and Luis Jaime Acosta, Editing by Rosalba O'Brien)