The fire and the shooting dead of two people by police broke a ceasefire between the government and protesters that was just hours old.
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Seven members of a family, including three children, have perished in a fire at their home in the Nicaraguan capital Managua.
It was one of the deadliest days of unrest in two months of confrontations between the government and protesters.
One of the victim's uncles accused the police of starting the fire - and a neighbour backed up his claim.
"We wanted to do anything humanely possible but the police didn't let us. They were attacked by the paramilitaries. They shot to kill," the neighbour, Jose Lopez, said.
Two people were shot dead by police in the unrest which came just hours after President Daniel Ortega and protesters struck an agreement to end hostilities.
The Secretary-General of the Organisation of American States tweeted that the fire was a "crime against humanity."
The protests began in April when President Ortega slashed pension benefits and protesters called on him to resign.
The Nicaraguan Center for Human Rights says at least 170 people have been killed.
Talks between the government and civic groups on democratic reforms which are being mediated by the Catholic Church will resume on Monday.