The Central American country's Roman Catholic bishops said earlier this week they would mediate in the so-called "national dialogue" scheduled to begin tomorrow.
Demonstrators and riot police clashed today in the northern town of Matagalpa, which is ruled by Ortega's Sandinista Front party.
Thirty-five people were wounded in the clashes and at least 10 people had been arrested, the Nicaraguan Association for Human Rights spokesman in the city, German Herrera, told local TV station 100% Noticias.
The education ministry said on its website that local high schools had been closed as a security precaution.
Ortega had accepted the notion of talks in the early days of the crackdown, but the church deemed he had not fulfilled conditions in which they could be held.
Among them is a visit by a regional human rights group which has finally been given permission to enter the country to investigate reports of widespread police brutality.
At least 53 people have been killed and some 400 injured in almost a month of protests, which initially broke out over proposed cuts to social security benefits, but morphed into widespread discontent with Ortega's leftist government.
The protests pose a serious challenge to the authority of Ortega, 72, who has ruled Nicaragua for the past 11 years and before that from 1979-1990, after overthrowing the dictatorship of Anastasio Somoza.
The protests that erupted on April 18 were the worst his government has faced, badly shaking his tight grip on power over the country, one of the poorest in Latin America. Ortega made a series of concessions after sharp domestic and international criticism over the use of security forces to put down the protests, and curbs on independent media to report them.
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