* At least five killed as police crack down

* Protesters flee stun grenades

* U.N. envoy says he will fight on

* Hundreds detained in Saturday crackdown

FEBRUARY 28

Myanmar police opened fired on Sunday on protests against military rule, killing at least four people and wounding several on the second day of a crackdown on demonstrations across the country, a doctor and a politician said.

A woman also died after police broke up a teachers' protest with stun grenades in the main city of Yangon, though the cause of her death was not known, her daughter and a colleague said.

Myanmar has been in chaos since the army seized power and detained elected government leader Aung San Suu Kyi and much of her party leadership on Feb 1, alleging fraud in a November election her party won in a landslide.

The coup, which brought a halt to tentative steps towards democracy after nearly 50 years of military rule, has drawn hundreds of thousands onto the streets and the condemnation of Western countries.

"Myanmar is like a battlefield," the Buddhist-majority country's first Catholic cardinal, Charles Maung Bo, said on Twitter.

Police opened fire in different parts of Yangon after stun grenades and tear gas failed to disperse crowds.

One man died after being brought to a hospital with a bullet wound in the chest, said a doctor at the hospital who asked not to be identified.

Police also opened fire in the southern town of Dawei, killing three and wounding several, politician Kyaw Min Htike told Reuters from the town.

The Irrawaddy online media outlet reported one person had been killed in the second city of Mandalay, where police also cracked down, while a charity reported two dead in the central town of Bago.

Police and the spokesman for the ruling military council did not respond to phone calls seeking comment.

Police also cracked down in the northeastern town of Lashio and Myeik in the deep south, residents and media reported.

Junta leader General Min Aung Hlaing said last week authorities were using minimal force to deal with the protests.

Nevertheless, at least five protesters have died in the turmoil. The army said a policeman has been killed.

The crackdown would appear to show a determination by the military to impose its authority in the face of widespread defiance, not just on the streets but more broadly in areas such as the civil service, municipal administration, the judiciary, education and health sectors and media.

'NEVER KNEEL'

In Yangon, several people were helped away, leaving blood-smeared pavements, after police fired, images posted by media showed.

Police also threw stun grenades, used tear gas and fired into the air, witnesses said. Nevertheless, hundreds of protesters refused to back down by early afternoon.

Some marched, some gathered to chant and sing and others set up barricades.

"If they push us, we'll rise. If they attack us, we'll defend. We'll never kneel down to the military boots," said Nyan Win Shein from one Yangon protest.

Police were out in force early in the day and moved swiftly to break up crowds. They dispersed the teachers' protest with stun grenades and one of them, Tin New Yee, later died. The cause of death might have been a heart attack, her daughter and another teacher told Reuters.

Police also hurled stun grenades outside a medical school in another part of the city, sending send doctors and students in white lab coats scattering.

A group called the Whitecoat Alliance of medics said more than 50 medical staff had been arrested.

State-run MRTV television said more than 470 people had been arrested on Saturday after police launched the nationwide crackdown.

It was not clear how many were detained on Sunday.

'INSTIL FEAR'

Youth activist Esther Ze Naw said earlier that people were battling the fear they had lived with for so long under military rule.

"It's obvious they're trying to instil fear in us by making us run and hide," she said. "We can't accept that."

The police action came after state television announced that Myanmar's U.N. envoy had been fired for betraying the country after he urged the United Nations to use "any means necessary" to reverse the coup.

The ambassador, Kyaw Moe Tun, had been fired, state media reported, though he remained defiant. "I decided to fight back as long as I can," he told Reuters in New York.

While Western countries have condemned the coup and some have imposed limited sanctions, the generals have traditionally shrugged off diplomatic pressure. They have promised to hold a new election but not set a date.

Suu Kyi's party and supporters said the result of the November vote must be respected.

Suu Kyi, 75, spent nearly 15 years under house arrest during military rule. She faces charges of illegally importing six walkie-talkie radios and of violating a natural disaster law by breaching coronavirus protocols.

The next hearing in her case is set for Monday.