Conservative politician Noel Vidaurre is accused of "undermining Nicaragua's independence" - a variant of the allegations of involvement in hostile foreign interference from the United States, which the presidential aspirants are suspected of - and has been placed under house arrest.
In addition to the presidential candidates, at least 21 politicians and opposition figures have been arrested or detained since the beginning of June.
Ortega has called the politicians criminals who want to overthrow the government.
The country goes to the presidential election in November and Ortega himself has not yet officially commented on his own candidacy.
However, the Speaker of Parliament said on Monday that Ortega would not only stand, but that his victory would be "indisputable".
Daniel Ortega was first elected president in 1984 and led the country until 1990. In 2007, he returned to power and has ruled the country ever since.
Since 2017, Ortega's wife Rosario Murillo has been the country's vice president.
Human rights organizations accuse the country's government of fabricating evidence against opposition groups, the United States has imposed sanctions on a number of Nicaraguans and calls the country's leader "a dictator".