Philippine government agents arrested the top executive of a news website critical of President Rodrigo Duterte's policies yesterday on a cyber libel complaint that she has dismissed as "baseless" and "baffling".

Officers sent by the National Bureau of Investigation (NBI) served Maria Ressa, chief executive of online news platform Rappler, with an arrest warrant issued by a local court on Tuesday at Rappler's headquarters, as her staff live-streamed a video of the incident on Facebook.

"We are not intimidated," Ressa told reporters, as she arrived at the NBI's main office.

"No amount of legal cases, black propaganda and lies can silence Filipino journalists who continue to hold the line," she said. "These legal acrobatics show how far the government will go to silence journalists, including the pettiness of forcing me to spend the night in jail."

The warrant stemmed from a complaint of cyber libel filed in 2017 by businessman Wilfredo Keng.

A Rappler story in 2012, updated in 2014, cited intelligence linking him to illegal activities, such as human and drug trafficking.

Ressa had argued that the statute of limitations on libel, which should also apply to cyber libel, had already expired, so the NBI had no basis for investigating and charging her with the offence.

Notwithstanding, since the Rappler article was published before the country's cyber crime law took effect, "there was yet no crime of cyber libel to be committed", she said.

Separately, Ressa and Rappler were indicted in October for allegedly attempting to evade taxes by not reporting gains of almost US$3 million (S$4 million) in the company's 2015 tax returns.

The penalties could include a fine, along with up to 10 years of imprisonment for Ressa.

She claimed these cases piling up against her and her start-up were "politically motivated, and they are manufactured".

But Mr Duterte's spokesman, Mr Salvador Panelo, told reporters the current case against Ressa was not based on any violation of press freedom. "Simply, she committed a crime and the court finds probable cause. And that's why she is now being charged," he said.

Mr Duterte has bristled over Rappler's coverage of his government: scrutiny over his brutal crackdown on the narcotics trade, the challenging of the accuracy of his public statements and criticisms of his foreign policies.

Mr Duterte banned one of Rappler's senior reporters from covering him, and lashed out at the media company in several public speeches.

In January last year, the Securities and Exchange Commission revoked Rappler's licence - though it allowed the company to continue operating as the decision was not yet final - for violating foreign ownership rules, a charge it denies.

The Philippine President's antagonism towards the media is not directed at Rappler alone. He had also threatened to block the licence renewal for ABS-CBN, the country's largest broadcast network.

He has called reporters "spies", used expletives to describe them and made thinly-veiled death threats, warning reporters they are not exempt "from assassination".

Following Ressa's arrest, in a letter addressed to President Duterte, international media watchdogs, World Association of Newspapers and News Publishers (WAN-IFRA) and the World Editors Forum expressed concern over the prosecution of Ressa.

The organisations also highlighted the "absurdity" of the arrest, stating that charging Ressa for an article that came out before the Cybercrime Prevention Act came into force, risked making the Act itself "unconstitutional" as it would be an "ex post facto law".

Reacting individually to Ressa's arrest, Vincent Peyregne, Chief Executive Officer of WAN-IFRA and Dave Callaway, President of the World Editors Forum called the incident an "outrageous escalation"

"This is an outrageous escalation in a growing war on the free press globally and specifically against a brave journalist," said Callaway.

Amnesty International called the cyber libel case "yet another absurd legal attack" that amounted to harassment.

Ressa was CNN's Jakarta bureau chief before setting out on her own and forming Rappler in 2012. She has won two prestigious journalism awards - a Press Freedom award from the New York-based Committee to Protect Journalists, and the International Centre for Journalists' Knight International Journalism Award.

Last year, Time magazine named her and three other journalists as its Persons of the Year.