A veteran Afro-Colombian politician who studied in the Soviet Union and fled his country after being kidnapped by a paramilitary group has been named Colombia’s next ambassador to the United States.

A leftist former rebel and a populist businessman have taken the top two spots among the six candidates in Colombia’s presidential election and are heading to a runoff showdown in June, delivering a blow to the country's political class.

A one-time rebel fighter has used an emotional anti-establishment discourse and promises to boost state involvement in the economy to garner a commanding lead in polls as Colombia heads into Sunday's presidential election.

Colombians have voted for a new congress and also cast ballots in presidential primaries to choose party candidates for the May presidential contest.