Lino Oviedo, a 69-year-old retired general, was running for Paraguay’s third largest party in April’s presidential election.
He had been involved in politics for decades, helping lead the coup which overthrew the military ruler Alfredo Stroessner in 1989.
Lino Oviedo was returning from a rally when his helicopter crashed north of the capital, Asuncion. An inquiry into the cause of the crash is under way.
The pilot and Lino Oviedo’s bodyguard also died. Police found their bodies in the province of Presidente Hayes.
Defence Minister Maria Liz Garcia said residents had heard a single explosion and the aircraft disintegrated. A storm had been reported along the flight path.
President Federico Franco has declared three days of mourning.
Lino Oviedo was running for the National Union of Ethical Citizens party (UNACE) in the presidential elections due on April 21.
As an army colonel he played a prominent part in the uprising which overthrew General Alfredo Stroessner in 1989, delivering the news that he was under arrest.
In the aftermath of the coup, Lino Oviedo rose quickly through military ranks, becoming brigadier-general and, by 1993, army chief.
His continued political campaigning, however, was criticized by then-president Juan Carlos Wasmosy, who asked him to step down as army chief in 1996 – an order he ignored.
He eventually stepped down and ran as a candidate for the Colorado Party in the 1998 presidential election.
However, before the election he was sentenced to 10 years in prison for plotting to overthrow President Juan Carlos Wasmosy. His running mate, Raul Cubas, was elected instead and ordered Lino Oviedo’s release.
But the killing of Vice-President Luis Maria Argana in 1999 and accusations by his relatives that Lino Oviedo had been behind the killing sent him into exile.
In 2004 he returned to Paraguay, where he was convicted over his 1996 insubordination. The conviction was overturned in 2007, allowing him to run in the 2008 election, which he lost.
Seen as a populist, Lino Oviedo often switched between Spanish and Guarani during his speeches.
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