Before the incident, the Red Cross said at least six people had died since protests began five days ago against government plans to increase worker contributions and lower pensions, causing a crisis for President Daniel Ortega.
The reform designed is aimed at settling a US$76 million deficit faced by the country's social security institute.
In a bid to calm the protests - the biggest of his 11-year presidency - Ortega agreed on Sunday to speak with the private sector about social security reforms, only to be rebuffed by Nicaragua's top private-sector business union.
The death toll from the protests rose to more than 20 people, a local human rights group said today.
Pope Francis called for an end to the violence.
"I'm worried about what's been happening in the last few days in Nicaragua, where after a social protest there were battles that have caused deaths," the Pontiff said during the Regina Coeli service at St Peter's Square.
"I express my solidarity with the country, and I join bishops in asking that the violence end, pointless spilling of blood is avoided and the underlying issues be resolved peacefully and with a sense of responsibility."
Journalists have reportedly faced attacks, been temporarily detained and had their work equipment stolen since the start of the protests. Meanwhile, four independent television outlets were taken off air on Thursday, although only one currently remains closed.