Nicaragua's government on Monday defended the arrests of 13 opposition figures, including four presidential hopefuls, claiming they are "usurpers" funded by the United States to topple President Daniel Ortega.

Five more opposition figures were detained on Sunday, including four from the Unamos opposition party.

The government said they had been detained on charges of "inciting foreign intervention" by the US.

Unamos, formerly known as the Sandinista Renewal Movement (MRS), is made up largely of dissidents who split from Ortega's Sandinista National Liberation Front (FSLN) because they disagreed with his leadership.

The opposition figures received "millions of dollars in cash from the American public though USAID," the government said in a document.

Nicaragua has come under fire internationally for the arrests, which began on June 2 when Cristiana Chamorro, the daughter of former president Violeta Barrios de Chamorro was held under house arrest.

Nicaragua President Daniel Ortega, pictured in 2018, has been in power since 2007 but is accused of authoritarianism Nicaragua President Daniel Ortega, pictured in 2018, has been in power since 2007 but is accused of authoritarianism Photo: AFP / INTI OCON

Chamorro is one of the four presidential hopefuls detained.

On Sunday, Julie Chung, the top US diplomat for Latin America, called the arrests "arbitrary" and denounced Ortega's "campaign of terror" in a tweet.

"OAS (Organization of American States) members must send a clear signal this week: enough repression. The region cannot stand by and wait to see who is next," she added.

Ortega's government and inner circle have been subject to US sanctions since a clampdown on demonstrations demanding his resignation in 2018 claimed at least 328 lives, according to rights groups.

Ortega governed Nicaragua from 1979 to 1990, returned to power in 2007 and has won two successive reelections since then.

Now 75, he is accused by the opposition and NGOs of increasing authoritarianism.

Ortega is widely expected to seek a fourth term in November elections, though he has not said so.