Nicaragua's Attorney General's Office on Tuesday charged an opposition leader with organized crime, terrorism and the deaths of four police officers within the context of a deadly sociopolitical crisis dating back to April 18.

The AG's office leveled those charges against farm leader Medardo Mairena, who is representing an opposition alliance in a national dialogue process.

Another farm leader, Pedro Mena, faces the same charges.

The AG's office also accuses Mairena and Mena, who were arrested last Friday, of being behind the murders of four police officers in the southern province of Rio San Juan.

Human rights activists gathered outside the Managua Central Judicial Complex, where the preliminary hearing took place, to demand the release of the peasant leaders.

They also held up signs that read "Justice for Small Farmers" and "Justice for Nicaraguans" and shouted slogans including "no more repression," "terrorist state" and "no to Ortega justice."

Four police and a teacher were killed last Thursday in the southeastern town of Morrito during an armed clash between law enforcement officers and individuals protesting President Daniel Ortega's government.

Humanitarian organizations have criticized Mairena's arrest, saying he is not a terrorist but a farm leader who has fought for five years against the Ortega government's plans to confiscate land (at below market value) to build a Chinese-funded interoceanic canal.

Also Tuesday, government forces took control of the opposition stronghold of Masaya - southeast of Managua - following an intense, seven-hour bombing campaign targeting the indigenous community of Monimbo that left at least three dead.

Nicaragua is experiencing its deadliest-ever peacetime socio-political crisis and the worst since the late 1980s when the country also was governed by the leftist Ortega. More than 350 people have died in the ongoing violence, humanitarian organizations say.

The protests against Ortega and his wife, Vice President Rosario Murillo, were triggered by a proposed welfare system overhaul and have continued even after those plans were scrapped on April 22.

Demonstrators have leveled accusations of abuse of authority and corruption against Ortega, in power for the past 11 years, and are calling on him to resign.