A Hong Kong photographer detained in Thailand for carrying a bulletproof jacket and helmet will face trial after pleading not guilty on Monday to a weapons possession charge.

The case has sparked an outcry from media freedom groups, who say journalists should not be punished for carrying body armour and protective gear in and out of dangerous zones.

Anthony Kwan Hok-chun, who works for the Hong Kong-based Initium media group, was briefly held on August 23 after trying to depart Bangkok's Suvarnabhumi airport with a flak jacket and helmet in his hand luggage.

Both items are classified as weapons under Thai law and Kwan was charged with breaching the Arms Control Act -- punishable by up to five years in jail.

He was in Thailand to cover the aftermath of August's deadly Bangkok bombing.

Kwan told reporters on Monday after the hearing at the Samut Prakan provincial court, near Bangkok, that he denied the charge.

"I'm not confessing... I don't think that a flak jacket is a weapon and I definitely didn't know it was illegal here," he said.

His plea means he will face a trial starting next month. The court granted Kwan bail and his lawyer later said he was free to leave the kingdom until the trial.

"I think everyone in my situation would be a little bit concerned. I'm preparing for the worst but hoping for the best," Kwan added.

Thailand's classification of protection equipment as a weapon that requires a license has been criticised by media groups, who say they are vital pieces of kit for reporters in a country where political violence routinely spills onto the streets.

Around 10 campaigners, mostly journalists, on Monday rallied in Hong Kong outside a building that houses Thailand's consulate calling for the case against Kwan to be dropped.

"This (his arrest) was unreasonable. Journalists face all kinds of situations and sometimes it can be dangerous. Anthony carried a bulletproof vest to protect himself and he was not the first one to do so," Ho Ka-tat, vice chairman of the Hong Kong Press Photographers Association, told AFP.

The Foreign Correspondents' Club of Thailand in a statement late Sunday also expressed disappointment at the charge.

"Thai authorities should consult with the media community... to explore a way around the 1987 law, which was surely not intended to prosecute journalists carrying out their normal duties," the statement said.