Authorities gave no details about the releases, which were announced by the Permanent Human Rights Commission (CPDH) after relatives posted images on social media of busloads of jubilant inmates leaving La Modela prison outside Managua.
"We have verified that a couple of minibuses left and some known prisoners were inside," CPDH president Marcos Carmona told AFP, adding that as many as 100 may have been released.
Many of the freed prisoners waved flags, sang the country's national anthem and shouted "Viva NIcaragua".
Relatives had spent the night outside the prison after rumors circulated that prisoners were to be released.
Ortega and the opposition were to begin talks Wednesday on easing tensions that began last April with deadly protests over the government's now-ditched pension reform.
A brutal crackdown by Ortega's security forces left 325 people dead and more than 750 arrested and accused of terrorism by the time the protests ended in October
Previous peace talks broke down last June after the president rejected opposition demands to step down and bring forward presidential elections.
Nicaragua is struggling with an economic crisis and a $315 million deficit. GDP shrank four percent last year and some economists say it could contract 11 percent in 2019.
The opposition accuses former guerrilla leader Ortega, in power since 2007, of establishing a corrupt dictatorship with his wife and vice president, Rosario Murillo.
Ortega's handling of the protests drew international condemnation and sanctions from the United States.