Nicaraguan police said Sunday that they had detained four more opposition figures in a roundup ahead of November presidential elections in which four would-be challengers of long-serving leader Daniel Ortega have already been held.

Those arrested Sunday were top figures of the Unamos opposition party -- its president Suyen Barahona Cuan, vice-president Hugo Torres, ex-guerilla Dora Maria Tellez and Ana Margarita Vigil Guardian, a police statement said.

It said the four were being investigated for "acts that undermine independence, sovereignty and self-determination, (and) inciting foreign interference in internal affairs," among other crimes.

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 charges stem from a law initiated by Ortega's government and approved by parliament in December to defend Nicaragua's "sovereignty," which has been criticized by opponents and rights bodies as a means of freezing out challengers.

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.

Among the latest detainees, Tellez, 65, has in recent years been a vocal critic of Ortega, a former comrade-in-arms.

Riot police stand guard outside the house of Nicaraguan opposition leader Cristiana Chamorro
Riot police stand guard outside the house of Nicaraguan opposition leader Cristiana Chamorro AFP / INTI OCON

They fought together as guerillas against the dictatorship of Anastasio Somoza in the 1970s, and she later served as his health minister in the 1980s, before leaving in 1995 to co-found the MRS.

She was fiercely critical of the Ortega's government clampdown on demonstrations that started in 2018 to demand his resignation, which according to rights groups claimed at least 328 lives.

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.

Since the beginning of the month, his forces have arrested about a dozen opposition figures, including four would-be presidential candidates, eliciting international condemnation and fresh US sanctions against Ortega allies.

Last month, Nicaragua's legislature appointed a majority of governing party-aligned magistrates to the election body that will oversee the vote.

It has since disqualified two parties from participating.