MANAGUA (Reuters) - Human rights advocates on Friday urged Nicaragua’s President Daniel Ortega to free 566 people still in custody that the opposition considers political prisoners following an initial release of detainees from a simmering political crisis.

FILE PHOTO: Demonstrators take part in a protest against Nicaraguan President Daniel Ortega's government at Central American University (UCA) in Managua, Nicaragua March 1, 2019.REUTERS/Oswaldo Rivas/File Photo

As it restarted talks with the opposition, the government on Wednesday released 100 people arrested during months of demonstrations against Ortega, the bloodiest conflict to shake the country since an 11-year civil war ended in 1990.

More than 320 people have died as pro-government forces cracked down on protests which erupted in April after an attempt to overhaul the pensions system. The resistance quickly swelled into a broad-based movement against Ortega’s rule.

Gradually crushing the rallies and street barricades, pro-government forces hunted down key protesters, detaining hundreds, many of whom were charged with terrorism. At least 30,000 Nicaraguans fled the country, many seeking asylum.

Talks to end the crisis first began last May but they quickly broke down amid opposition calls for early elections and frustration over a lack of concessions by the government.

Agreement has yet to be reached after two days of talks behind closed doors on the outskirts of Managua. The government has not accepted the mediation of the Organization of American States (OAS) nor of the United Nations, rights advocates said.

The activists said none of the prisoners released were among the most prominent, including journalists Miguel Mora and Lucia Pineda, who have been accused of terrorism.

Another emblematic figure who remains in custody is Medardo Mairena, a farmworkers’ leader who was sentenced in December to 216 years in prison for terrorism, murder and organized crime.

“The state’s afraid. It’s afraid of freeing leaders or the farmers who are detained,” said Braulio Abarca, of Nicaragua Nunca Mas, a group of rights activists based in exile.

Maja Kocijancic, a spokeswoman for a European Union delegation in Nicaragua, said: “We hope that all prisoners illegally detained will be released. Respect for fundamental rights, especially freedom of expression and assembly, is essential to restore confidence.”

Protesters criticize Ortega, a former Marxist guerrilla who has held the presidency since 2007, and Rosario Murillo, his wife and vice-president, for their ever-tightening grip on power in what is one of the poorest countries in the Americas.

Ortega denies holding political prisoners. His government says the protests were an attempted coup, that the judiciary is independent and that those behind bars have committed crimes.