One of Nicaragua’s most prominent journalists has accused the “criminal dictatorship” of the country’s president, Daniel Ortega, of launching a brazen attack on the press after police raided, ransacked and commandeered his newsroom in the latest chapter of an escalating crackdown on dissent.

Carlos Fernando Chamorro, the editor of Confidencial, a combative newsletter and website, said officers had stormed its headquarters in the capital Managua late on Thursday night, seizing laptops and computers, and had returned at about 10.30pm on Friday to occupy the premises.

When Chamorro tried to visit his newsroom on Saturday morning, he found it guarded by armed police. He demanded to see documents explaining why it had been seized, but was ignored.

Carlos Fernando Chamorro walks through his ransacked offices. Photograph: Alfredo Zuniga/AP

“They have taken our newsroom … They are physically closing down our offices by taking them militarily,” Chamorro said by telephone on Saturday morning.

“The national police has been transformed into a delinquent force by its supreme chief, the dictator Daniel Ortega,” Chamorro told the guards, denouncing their “absolutely illegal occupation”.

The Nicaraguan writer Gioconda Belli was also present. She attacked Ortega’s “intolerable” attack on the press and democracy. “This is an act of revenge,” she told Confidencial.

Later, as Chamorro and a group of journalists and supporters sought answers at the police headquarters in Managua, they were driven away by armed riot police.

“Orteguismo knows only how to respond with violence,” tweeted Confidencial journalist Yader Luna. “It’s the only thing keeping it in power: brute force.”

He called the operation a calculated attempt to silence critics by Ortega, whose role in the 1979 Sandinista revolution and cold war struggle with the US made him an icon for the global left.

“We have been in this regime’s sights for many years. They have attacked us, they have pressured us, they have intimidated us and they have spied on us, and the only conclusion I can draw is that they are now moving towards what they consider the coup de grâce,” Chamorro said. “It is a blow and a warning.”

Nicaraguan president Daniel Ortega. Photograph: Yamil Lage/AFP/Getty Images

Activists see the raid on Confidencial as part of an intensifying blitz against Ortega’s political foes, who have been seeking to force the 73-year-old from power since mass student-led protests erupted in Managua early last year.

The demonstrations swelled into a nationwide revolt that for a time appeared that it might topple Ortega, but that the security forces and heavily armed gangs of masked pro-government paramilitaries eventually put down.

Authorities have tightened the screws on the opposition in recent weeks, outlawing street demonstrations and targeting human rights NGOs that sought to document the human cost of the political upheaval, which has claimed more than 325 lives and forced thousands into exile.

Chamorro, who is the son of Nicaragua’s first female president and a member of one of its most influential families, vowed to stay in Nicaragua reporting on the crisis with his team of talented young journalists. “I’m here and here I will remain … This will not stop us doing journalism, it will not prevent us from continuing to denounce and investigate.”

He said he felt optimistic about the future of Central America’s largest country despite what he called its drift into authoritarian rule under Ortega.

“This criminal dictatorship is unsustainable,” he said. “This escalation of repression is an expression of its extreme political frailty. The only way it knows how to govern is through terror, and we know that a regime which governs through terror will collapse. The economy is already collapsing and this is provoking the collapse of the country.

“How long it will take for this dictatorship to end? I don’t know. But I believe they are already defeated. The regime has no way out.”

The homepage of Nicaragua’s state-run propaganda outlet El 19 made no mention of the raid on Saturday but did find space for five photographs of Ortega and a 3,400-word essay by his vice-president and wife, Rosario Murillo.

“Nicaragua flourishes in love, work, security and peace!” she wrote.