"On Saturday, we are going to summon people to protest. The Libre (Party) and Xiomara (Castro) have been robbed of their victory, and we are going to show it," her husband, ousted ex-president Manuel Zelaya, told Radio and TV Globo.
The country's Supreme Electoral Tribunal has not declared an official winner in Sunday's vote.
But it has said that conservative Juan Orlando Hernandez has an irreversible lead of 34 percent over Castro, at 29 percent, with tallies still incomplete.
Castro and her husband allege that the Supreme Electoral Tribunal manipulated 19 percent of the votes to favour Hernandez.
Also Read
European Union and Organisation of American States observers called the voting process transparent and non-problematic.
Castro, who proposes "Honduran-style democratic socialism," wants to rewrite the constitution and "re-found" the country -- a move similar to the one that led to the coup that ousted her husband in 2009.
Zelaya was elected Honduran president as a PL candidate in 2005.