Several former allies of long-time President Daniel Ortega were among those arrested on Sunday.
Police accused them of inciting foreign interference in Nicaragua’s affairs, among other crimes.
About 12 opposition figures, including four hopefuls in November’s election, have been arrested in recent days.
Mr Ortega, 75, is expected to seek a fourth consecutive term in November’s presidential election. But opinion polls suggest his popularity has plummeted after a violent crackdown on anti-government protests in 2018 in which hundreds of people were killed..
In power since 2007, Mr Ortega has been accused of suppressing dissent and political opposition to his rule ahead of the poll.
The police said those held on Sunday were all members of Unamos, an opposition party that is largely made up of dissidents who split from President Ortega’s ruling party, the Sandinista National Liberation Front (FSLN).
The FSLN was a socialist revolutionary movement that swept to power in the Central American nation in 1979.