Ressa spent a night in detention after authorities arrested the veteran reporter at her Manila office Wednesday in a sharp upping of government pressure on her and her website Rappler.
The site and Ressa, 55, have been hit with tax evasion charges and now a libel case after clashing repeatedly with Philippine President Rodrigo Duterte over his deadly crackdown on narcotics that has killed thousands.
"It's about two things: abuse of power and weaponisation of the law," an emotional Ressa told journalists as she stepped out of a Manila court where she posted bail.
"What we are seeing is death by a thousand cuts to our democracy," added Ressa, who was named a Time Magazine "Person of the Year" in 2018 for her journalism.
Ressa paid 100,000 pesos ($1,900), the sixth time she posted bail to avoid detention following a slew of charges.
International condemnation from dignitaries and press freedom and human rights groups has poured in since plainclothes agents appeared at Rappler to serve an arrest warrant on the charge that carries up to 12 years behind bars.
"The arrest of Maria Ressa is an outrage," said Committee to Protect Journalists Board Chair Kathleen Carroll. "She should be freed immediately and the Philippines government needs to cease its multi-pronged attack on Rappler."
Former US Secretary of State Madeleine Albright also stood by Ressa.
"The arrest of journalist @mariaressa by the Philippine government is outrageous and must be condemned by all democratic nations," Albright said in a tweet where she called Ressa a friend.
Duterte attacks on media
The libel case against Ressa and former Rappler reporter Reynaldo Santos, Jr. stems from a 2012 report written about a businessman's alleged ties to a then-judge on the nation's top court.
While investigators initially dismissed the businessman's 2017 complaint about the article, the case was subsequently forwarded to prosecutors for their consideration.
The legal foundation of the case is a controversial law aiming to crack down on online offences ranging from harassment to child pornography.
Ressa's team has argued the legislation did not take effect until months after the story was published and is not retroactive, however the government has countered that it is fair game because the story was updated in 2014.
"In essence in the contemplation of the law it is a new article because of the modification, republication," Markk Perete, spokesman for Department of Justice prosecutors, told AFP. "That is deemed as a new article."
Rappler concedes the story was updated, but notes it was to fix a typo and no substantive changes were made.
The businessman who sued Rappler, Wilfredo Keng, on Thursday welcomed the charges as he said the website "destroyed my reputation and endangered my life".
Duterte has lashed out at other critical media outfits, including the Philippine Daily Inquirer newspaper and broadcaster ABS-CBN.
He had threatened to go after their owners over alleged unpaid taxes or block the network's franchise renewal application.
Some of the drug crackdown's highest-profile detractors have wound up behind bars, including Senator Leila de Lima, who was jailed on drug charges she insists were fabricated to silence her.
Ressa insists the site is not anti-Duterte, saying it is just doing its job to hold the government to account.