French police Thursday released journalist Ariane Lavrilleux, who was arrested after reporting on leaked documents that claimed French intelligence was used to target civilians in Egypt, in a two-day detention that alarmed rights groups, she said.

Investigative website Disclose published a series of articles in November 2021 based on hundreds of secret documents.

It said they showed how information from a French counter-intelligence operation in Egypt, codenamed "Sirli", was used by the Egyptian state for "a campaign of arbitrary killings" against smugglers operating along the Libyan border.

On Tuesday, Lavrilleux's home was searched and she was arrested for questioning by agents of the DGSI, France's domestic intelligence agency, Disclose announced on X (formerly Twitter).

It denounced an "unacceptable attack on the secrecy of sources" -- a view quickly backed by the Society of Journalists and Reporters Without Borders (RSF).

"We fear that the DGSI's actions will undermine the secrecy of the sources," RSF said.

Amnesty International's secretary general Agnes Callamard said: "It is deeply chilling that, almost two years after the revelations that France was allegedly complicit in the extrajudicial executions of hundreds of people in Egypt, it is the journalist who exposed these atrocities that is being targeted, rather than those responsible."

Journalists had held a protest outside the police headquarters where she was held.

Lavrilleux said on X she had been freed, posting a jubilant picture and the hashtag #JournalismisNotaCrime. The maximum people can be held without charge in France is 48 hours.

Virginie Marquet, lawyer for Lavrilleux and Disclose, confirmed she had been released, without any charge at this stage. A former member of the military also detained Tuesday will however appear before a magistrate to face further prosecution, another source said.

The initial Disclose articles said French forces were complicit in at least 19 bombings against smugglers between 2016 and 2018 in the region.

The documents showed there were warnings from officials within the French government, but the operation was not called into question, Disclose said.

France's Ministry of the Armed Forces filed a complaint for "violation of national defence secrecy" following the publication of the article, and a case was opened in July 2022 by the Paris prosecutor's office that was then placed in the hands of the DGSI.