Vice-President Rosario Murillo blamed the violence on "vampires", but said the government was open to dialogue.
It is the deadliest wave of unrest in Nicaragua since President Daniel Ortega came to office in 2007.
It follows legislation that increased pension contributions for workers and employers and reduced overall benefits.
The violence began on Wednesday, when pensioners took to the streets in the capital Managua. They were joined the next day by thousands of students and workers.
At least 100 people have been injured. The dead include two protesters and a policeman who were killed in Managua on Friday, after demonstrations turned violent.
The unrest continued into Friday night in several cities.
Vice-President and First Lady Rosario Murillo compared the protesters to "vampires demanding blood to feed their political agenda".
But she added that "dialogue remains open" and "all subjects are on the table".
The Sandinista government says public buildings have been set on fire. Students from Polytechnic University in Managua have barricaded their campus.
Protesters have accused riot police and government supporters of initiating the violence. Independent TV stations say they have been taken off-air after broadcasting the demonstrations live.
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