The editor of one of Myanmar's top newspapers will face trial for defaming the military by posting a satirical article on Facebook, under a controversial law critics say is stifling press freedom.

Last month the army sued the editor in chief of The Voice, Kyaw Min Swe, for uploading a link to an article poking fun at the military's leaders for sitting around talking while ordinary soldiers are killed.

On Friday a court in Yangon sent him to the city's notorious Insein prison to await the start of his trial next week.

Hopes had been high that Myanmar's first freely elected government in generations would usher in a new era of free speech when they took power last year after half a century of military rule.

But defamation prosecutions have soared since Aung San Suu Kyi's National League for Democracy (NLD) came to power, with social media satirists, activists and journalists increasingly targeted.

Activists say various defamation laws are being used to stifle free speech and stop the media from criticising the government.

Kyaw Min Swe is being prosecuted under the country's broadly-worded telecommunications law -- known as "66(d)" -- which forbids uploading false of defamatory information online.

If convicted he faces up to six months in jail.

The writer of the article, Kyaw Swa Naing, was acquitted by the court on Friday because he did not actually upload his piece.

Speaking to reporters outside court, he said the law "must be amended".

"Journalists are being harmed very much. Although we do not know what the verdict will be for brother Kyaw Min Swe... he's now being held in prison," he added.

Dozens of journalists and free speech activists wearing white armbands and with painted faces protested near the court on Friday, displaying banners emblazoned with "press freedom" in red.

"The government is letting people and organisations sue journalists... using a (law) that should be abolished under the NLD," protesters Phyo Wai said.

The Voice article at the centre of the case poked fun at a military propaganda film called "Pyi Daung Su Thit Sar" (Faithful to the Union) lauding the army's victories over armed ethnic groups.

It was released just weeks before the start of the government's latest round of tense peace negotiations with Myanmar's myriad insurgent groups, and as some of the heaviest fighting in decades rages in border areas.