The head of a Philippine news website that has locked horns with President Rodrigo Duterte was freed on bail on Thursday, a day after her widely condemned arrest on libel charges.

Maria Ressa, the award-winning head of news platform Rappler, was served a warrant live on television at her office on Wednesday for what media watchdogs said were trumped-up charges aimed at intimidating those who challenge Duterte's rule.

"For me it's about two things - abuse of power and weaponisation of the law," Ressa said.

"You have to express outrage and do it now. Press freedom is not just about journalists, is not just about us, it's not just about me, it's not just about Rappler.

"Press freedom is the foundation of every single right of every Filipino to the truth," she told reporters after posting bail.

Ressa is accused of cyber libel over a 2012 Rappler article, which was updated in 2014, that linked a Filipino businessman to murder, human trafficking and drug smuggling. Rappler cited information contained in a 2002 intelligence report, but did not say which agency compiled it.

The businessman's lawyer says the information was wrong and that the article was defamatory and that he wants to clear his name.

Duterte has made no secret of his dislike of Rappler and has sparred frequently with its reporters, who are known for scrutinising his policies and appointments and for questioning the accuracy of his sweeping, often bellicose, statements.

Rappler's reporting has accused his administration of creating a social media "ecosystem" designed to defend Duterte, threaten and discredit his opponents, and discourage Filipinos from criticising him for fear of being attacked by online trolls. The government denies the accusations.

Duterte once suggested Rappler was American-owned and could be linked to the U.S. Central Intelligence Agency. He called Rappler a "fake news outlet" and banned a reporter from covering his events.

Rappler, founded in 2010, is no stranger to legal challenges and faces ongoing cases over tax evasion and alleged ownership violations. It denies wrongdoing and says it will not be cowed into self-censorship.

This was the first time Ressa was arrested, although she said her release marked the sixth time she had been charged and granted bail.

Duterte's spokesman, Salvador Panelo, said the libel case had nothing to do with the government and that Duterte had no interest in punishing journalists.

"That's absolutely unrelated. The president has been criticised and he does not bother," he told DZMM radio.

Ressa spent the night in a room at the National Bureau of Investigation headquarters in the capital, Manila, and was unable to post bail because the court was closed.

Her brief detention was criticised by journalists and activists at home and abroad. Former U.S. Secretary of State Madeleine Albright said it was an "outrageous" arrest that "must be condemned by all democratic nations".

London-based Amnesty International said Duterte's administration was using the law to "relentlessly intimidate and harass journalists." Dabet Panelo of the National Union of Journalists of the Philippines said it would backfire and called for protests on Friday.

- Reuters