ASUNCION, Paraguay -- Paraguayan presidential candidate Lino Cesar Oviedo has been killed in a helicopter crash, authorities said Sunday.

Oviedo was returning with his bodyguard from a political rally Saturday night in northern Paraguay when his pilot encountered bad weather. All three were killed in the crash, said Johnny Villalba, a spokesman for Paraguay's airport authority.

"The causes of the accident and other details are being investigated by experts from the Center for Investigations and Prevention of Air Accidents," he said, an agency that is part of Paraguay's airport authority.

Oviedo, 69, was running in April's elections as leader of Paraguay's third-largest opposition party, the National Union of Ethical Citizens. A retired general and former army chief, Oviedo had tried for years to lead his nation, and not always through democratic means.

A populist with support among Paraguay's poor indigenous majority, Oviedo was convicted of plotting to overthrow President Juan Carlos Wasmosy in a short-lived rebellion in April 1996, and served half of a 10-year sentence before being released.

The Supreme Court later exonerated Oviedo after military officers denied there had been a coup attempt, freeing him to run for president in 2008. He came in third, splitting the vote that gave former Catholic bishop Fernando Lugo the presidency and ended 60 years of one-party rule by the Colorado party.