Colombians will vote on Sunday in a presidential run-off that former Defense Minister Juan Manuel Santos was expected to win after vowing to continue the incumbent president's policies on security and the economy.

Santos easily won the first round in May and holds a big lead in opinion polls over ex-Bogota mayor Antanas Mockus, a former mathematics professor who issued a challenge to traditional parties with a call for cleaner government.

President Alvaro Uribe, a conservative U.S. ally, steps down in August after battering leftist rebels who once controlled large parts of Latin America's No. 4 oil producer, now seen as an attractive destination for foreign investment.

Santos will guarantee strong policy continuity over the next four years with a solid congressional base of support to start with, although governance could prove more challenging over time, said Eurasia Group analyst Patrick Esteruelas.

Support for Santos has continued to grow since he almost beat Mockus outright in the first round, and the latest Invamer-Gallup opinion poll said he could win 66.5 percent of votes, compared with 27.4 percent for Mockus.

Investors broadly view a victory by Santos as a continuation of Uribe's security and pro-business policies, and say a Santos win will maintain favorable support in the short term for the peso currency and local TES bonds.

Whoever wins on Sunday will inherit a much safer country than when Uribe came to power in 2002, but will have to tackle the region's highest unemployment rate, a stubborn fiscal deficit and tensions with neighboring Venezuela, where a trade dispute is weighing on Colombia's economic recovery.

Nearly half of 30 million eligible voters cast ballots in the May 30 first round, but analysts say that figure could drop by as much as 10 percent because of Santos's lead and the competing attraction of World Cup soccer matches.

Once written off as a failing state, Colombia has seen its long war ebb as Uribe used billions of dollars of aid from Washington to send soldiers backed by helicopters and better military intelligence to drive back rebels.

With the economy pulling back from the global crisis, polls show voters are less concerned about security, more concerned with jobs and healthcare, and weary over human rights and graft scandals that tarnished Uribe's second term.

The differences between Santos and Mockus are only adjective, secondary, and individualist. In essence, both characters embody the same lost policy: haggard Uribismo, said Esteban Carlos Mejia, a columnist for the El Espectador paper.

(Editing by Jack Kimball and Xavier Briand)