Myanmar's military authorities have charged an Associated Press photographer and five other journalists over their coverage of anti-coup protests, their lawyer said on Wednesday.

AP photographer Thein Zaw, 32, was arrested on Saturday as he covered a demonstration in Myanmar's commercial hub Rangoon.

The country has been in uproar since February 1, when the army detained Aung San Suu Kyi and other civilian leaders, ending Myanmar's brief experiment with democracy and sparking protests far and wide.

Thein Zaw's lawyer said he and five other Myanmar journalists had been charged under a law against "causing fear, spreading false news or agitating directly or indirectly a government employee".

The junta amended the law last month, to increase the maximum sentence from two years to three years in jail.

"Ko Thein Zaw was simply reporting in line with press freedom law -- he wasn't protesting, he was just doing his work, the lawyer, Tin Zar Oo, said, adding that all six were being held at Insein prison in Rangoon.

The other five journalists are from Myanmar Now, Myanmar Photo Agency, 7Day News, Zee Kwet Online news and a freelancer, according to AP.

AP's vice-president of international news Ian Philips called for Thein Zaw's immediate release.

"Independent journalists must be allowed to freely and safely report the news without fear of retribution," he said.

"AP decries in the strongest terms the arbitrary detention of Thein Zaw."

Since the coup, authorities have steadily stepped up their tactics against anti-military protesters, deploying tear gas, water cannon and rubber bullets, as well as isolated incidents of live rounds.

Sunday was the bloodiest day since the military takeover, with the UN saying at least 18 protesters were killed across the country. AFP independently confirmed 11, adding to five killed in earlier incidents.

According to the Assistance Association for Political Prisoners (AAPP) monitoring group, more than 1,200 people have been arrested since the coup, with about 900 still behind bars or facing charges.

But the real number is likely far higher -- state-run media reported that on Sunday alone more than 1,300 people were arrested.

AAPP says that 34 journalists are among those detained, with 15 released so far.

"This repression is obstructing the flow of accurate information and news," AAPP said, adding that journalists were being subjected to "violent attacks" despite having clear credentials.

The most recent confirmed arrest came Monday, when a Myanmar journalist with broadcasting service Democratic Voice of Burma (DVB) live-streamed a late-night raid on his home.

The footage -- posted on DVB's Facebook page -- appeared to show loud bangs outside his apartment building as he pleaded with authorities not to shoot.