Myanmar's junta late Sunday imposed martial law in two densely populated Yangon townships after at least 18 protesters were killed in one of the deadliest days since the February 1 coup.

Sunday's violence brings the number of people killed in mass protests since the military wrenched civilian leader Aung San Suu Kyi from power to around 100, though activists and rights groups believe it could be higher.

The State Administration Council (SAC) has repeatedly justified its power grab by alleging widespread electoral fraud in November's elections, which Suu Kyi's National League for Democracy (NLD) party won by a landslide.

State-run media announced late Sunday that Yangon's massive Hlaing Tharyar township and the neighbouring Shwepyitha township will be placed under martial law.

The vast and impoverished townships are known as factory hubs and home to garment factories.

The junta "gives administrative and judicial martial law power to the Yangon regional commander... to perform security, maintain the rule of law and tranquility more effectively," said an announcer on state-run TV.

Soldiers and police have in recent weeks been staging near-daily crackdowns against demonstrators calling for a return to democracy, using tear gas and firing rubber bullets and live rounds to quell anti-coup protests.

In Hlaing Tharyar township police and soldiers clashed violently, with protesters wielding sticks and knives and rushing for protection behind makeshift barricades.

Protesters using cut-out trash cans as shields managed to rescue some demonstrators wounded when the security forces opened fire, but a doctor said not all could be reached.

"I can confirm 15 have died," the doctor told AFP, adding that she had treated about 50 people and expects the death toll to climb.

The Assistance Association for Political Prisoners monitoring group -- which verifies arrests and fatalities since the coup -- gave a higher death toll.

Residents hiding at home heard gunshots continuously throughout the day, while military trucks were seen driving through the smoky streets.

- 'I will fight until the end' -

Hours before violence erupted in Hlaing Tharyar a police officer posted a TikTok video saying in a voiceover that security forces would be bringing heavy weaponry into the township.

"I will not have mercy on Hlaing Tharyar and they will fight back seriously too because there are all kinds of characters there," said the officer under the account @aungthuraphyo40.

"We should not have mercy on them."

The video, which was seen and verified by AFP fact checkers, was removed hours later.

© AFP