BOGOTA - Gustavo Petro will on Sunday become Colombia's first leftist president, elected by voters who hope he can carry out ambitious social and economic reforms meant to reduce violence and deep inequality in the polarized Andean country.

Petro, a former member of the M-19 guerrillas, is set to be inaugurated in Bogota's Bolivar Plaza on Sunday afternoon.

Senate president Roy Barreras will swear him in front of some 100,000 people, including Spanish King Felipe VI, at least nine Latin American presidents and regular Colombians invited by Petro.

New Vice President Francia Marquez, an environmental activist and former housekeeper, will be the first Afro-Colombian woman to hold her post.

Petro, a 62-year-old former senator, has said his first priority will be actions to fight hunger in the country of 50 million, where nearly half the population lives in some kind of poverty.

A $5.8 billion tax reform, which would raise duties on high earners to fund social programs, will be proposed to congress on Monday by the new Finance Minister Jose Antonio Ocampo.

"I'm also nervous about becoming president," Petro recently told students at his alma mater Externado University in Bogota, when asked about the challenges he faces.

Petro has pledged free public university education and healthcare changes and constructed a broad congressional coalition of leftist and centrist parties to pass his platform.

Promises of a pension reform and a halt to new oil development have caused investor jitters despite the appointment of Ocampo, a long-time official, as finance minister.

The new president, a former mayor of Bogota, has also promised to revive scuppered peace negotiations with the National Liberation Army (ELN) rebels and apply a 2016 peace deal to ex-members of the FARC guerrillas who reject it.

His foreign minister has said the government will hold dialogue with gangs and potentially give members reduced sentences in exchange for information about drug trafficking.

The sword of Latin American liberation hero Simon Bolivar - stolen by Petro's erstwhile M-19 comrades in 1974 - will be on display in the plaza.

"We all have to wish each other good luck," Petro told the students.

(Reporting by Luis Jaime Acosta; Writing by Julia Symmes Cobb Editing by Marguerita Choy)