Seven people were killed in the capital Managua and one in Masaya and a baby was among the dead, said Georgina Ruiz, an activist with the Nicaraguan Center for Human Rights (CENIDH).
The rights group says more than 200 people have been killed in protests that started just over two months ago demanding President Daniel Ortega step down.
Starting after midnight, police and paramilitary forces flooded six neighborhoods in the east of Managua, as well as the National Autonomous University of Nicaragua (UNAN) where scores of students are holed up. Among the dead were two students killed in the university area and two minors who died in Managua - a 17-year-old and a baby who was shot in the head.
The baby was killed when his mother was taking him to a babysitter. "He was killed by a police gunshot. I saw them. They were police," the mother Kenia Navarrete told news channel Cien por Cien Noticias.
The government denied the charge, saying criminals in the university area were to blame.
UNAN is one of several student protest camps in Managua. About 450 students have been living there under plastic tarp tents and in class buildings, surrounded by empty bottles, old food and used rounds from their homemade mortars.
"Ortega's government continues to repress and kill young people," CENIDH said on Twitter.
According to Alvaro Leiva, secretary of the Nicaraguan Pro-Human Rights Association, the attack against the university was intended "to plant terror in the population" ahead of a march planned for Saturday afternoon in memory of victims of the violence.
Later Saturday organizers cancelled the march due to what they branded "indiscriminate" attacks by government forces. The organizing Civil Alliance opposition called for a 48-hour strike among social sectors and trade unions to press for Ortega's departure.
The Nicaraguan Episcopal Conference urged Ortega's government and the opposition to return to the negotiating table on Monday to discuss a proposal to bring elections forward from 2021 to March 2019, in a bid to end the crisis.
Ortega, 72, has not responded to the Catholic bishops' initiative, but has previously expressed his willingness to work toward democratization.
Talks between the government and the opposition Civic Alliance were suspended once again last Monday when the government failed to allow international human rights bodies to investigate the violence.
Protests erupted on April 18 against now-scrapped social security reforms, but have grown into demands for justice for those killed and the exit of Ortega and his wife and vice president Rosario Murillo.
Ortega, a former leftist guerrilla, led the country from 1979 to 1990 and then returned to the presidency in 2007. He is now serving his third consecutive term.
