Nicaragua’s best-known journalist has gone into exile after armed police raided and ransacked his newsroom in what experts called the latest chapter of the country’s slide into autocracy under President Daniel Ortega.

Carlos Fernando Chamorro, the editor of Confidencial, a combative newsletter and website and a member of one of Nicaragua’s most influential families, announced his decision on Sunday.

Chamorro wrote: “I will continue to fulfil my duties as a journalist from Costa Rica … investigating and denouncing the crimes, corruption and the impunity and documenting the terminal crisis of this dictatorship.”

He blamed his decision on the “extreme threats” he and other Nicaraguan journalists were facing as a result of an intensifying press crackdown being waged by the country’s Sandinista leader.

Carlos Fernando Chamorro walks through the ransacked offices of Confidencial, which was raided by police in December. Photograph: Alfredo Zuniga/AP

He claimed leaving Nicaragua – where Ortega has been struggling to regain control since a destabilising and deadly period of political upheaval began last April – was the only way to ensure his “physical integrity and freedom”.

Throughout Nicaragua’s ongoing crisis, Chamorro has repeatedly insisted he would not be forced from the country.

“I’m here and here I will remain,” he told the Guardian last month after his offices in the capital, Managua, were stormed and occupied by police. However, the assault on the media has continued to gather pace despite international condemnation.

A week after the raid on Chamorro’s offices, another channel, 100% Noticias, was stormed and two of its directors taken into custody, where they remain. The Committee to Protect Journalists labelled the operations “an unacceptable escalation of the Nicaraguan government’s crackdown on the country’s independent media”.

Last week, Nicaragua’s biggest newspaper, La Prensa, printed an almost blank front page in protest at the government’s apparent attempt to silence it by impounding printing materials.

A mural in Managua of Nicaragua’s president, Daniel Ortega, defaced with the word ‘murderer’. Photograph: Esteban Félix/AP

Readers used marker pens and pencils to write their own headlines into the empty space in a viral social media campaign that used the hashtag #LaPrensaChallenge.

Suggested front-page headlines included: “They want to silence us but you can’t silence the truth”, “SOS Nicaragua” and “Nicaragua declares itself a territory free of Orteguismo”.

Another headline celebrated the nine-month-old anti-Ortega movement with the rallying cry: “Insist, persist, resist and never desist!”

In the article announcing his exile, Chamorro called on Nicaraguan citizens to continue using social media to challenge Ortega and vowed to continue his own journalism from Costa Rica, home to a sizeable and growing Nicaraguan exile community.

“I am confident that better days lie ahead for Nicaragua,” he wrote.