Myanmar's military raided the Yangon headquarters of ousted leader Aung San Suu Kyi's party late Tuesday, officials said, as the United States joined the UN in "strongly" condemning violence against protesters demanding a return to democracy.

The latest assault on Myanmar's civilian leadership came as anger at last week's coup and the detention of Suu Kyi by the generals has driven hundreds of thousands of people into the streets, defying a junta ban on rallies.

Myanmar military chief General Min Aung Hlaing declared that this time, things would be "different" from the army's previous 49-year reign, which ended in 2011 Myanmar military chief General Min Aung Hlaing declared that this time, things would be "different" from the army's previous 49-year reign, which ended in 2011 Photo: Myawaddy TV / -

"The military dictator raided and destroyed NLD headquarters at around 9:30 pm," the National League for Democracy announced on its Facebook page.

The party's short statement gave no further details.

Myanmar's generals have imposed a ban on gatherings of more than five people in protest flashpoint sites Myanmar's generals have imposed a ban on gatherings of more than five people in protest flashpoint sites Photo: AFP / YE AUNG THU

The raid came after demonstrations erupted for a fourth straight day Tuesday, with police using water cannon in several cities, firing rubber bullets at protesters in the capital Naypyidaw and deploying tear gas in Mandalay.

The rallies came despite a warning from the junta that it would take action against demonstrations that threatened "stability", and a new ban on gatherings of more than five people.

Photo: AFP / John SAEKI

The United States, which has led global censure of the coup, on Tuesday renewed its call for freedom of expression in Myanmar -- and for the generals to step down.

"We strongly condemn violence against demonstrators," US State Department spokesman Ned Price told reporters, adding that people in Myanmar "have rights to peaceful assembly."

Since the junta staged a coup on February 1 and ousted Myanmar's leader Aung San Suu Kyi from power, waves of dissent have swept the country -- with hundreds of thousands amassing in major cities Since the junta staged a coup on February 1 and ousted Myanmar's leader Aung San Suu Kyi from power, waves of dissent have swept the country -- with hundreds of thousands amassing in major cities Photo: AFP / STR

"We repeat our calls for the military to relinquish power, restore democratically elected government, release those detained and lift all telecommunication restrictions and to refrain from violence," he said.

Price has previously said US requests to speak to Suu Kyi had been denied.

Protesters face off with police standing guard on a road during a demostration against the military coup in Naypyidaw on February 8 Protesters face off with police standing guard on a road during a demostration against the military coup in Naypyidaw on February 8 Photo: AFP / STR

As night fell, the United Nations also voiced its "strong concern" over the violence.

Anti-coup protesters rally across in Yangon on Tuesday, defying warnings from the military threatening "action" against large gatherings. Anti-coup protesters rally across in Yangon on Tuesday, defying warnings from the military threatening "action" against large gatherings. Photo: AFPTV

"The use of disproportionate force against demonstrators is unacceptable," said Ola Almgren, the UN resident coordinator and humanitarian coordinator in Myanmar.

European Union foreign policy chief Josep Borrell said Tuesday the bloc could impose fresh sanctions on Myanmar's military, but warned any punishment should not hit the population.

A protester wears a sign with a hashtag #justiceformyanmar as they take part in a demonstration against the military coup in Yangon on February 8 A protester wears a sign with a hashtag #justiceformyanmar as they take part in a demonstration against the military coup in Yangon on February 8 Photo: AFP / YE AUNG THU

"We are currently reviewing all our options," Borrell told lawmakers at the European Parliament.

In Naypyidaw, the remote capital purpose-built by the previous military regime, witnesses said police fired projectiles at protesters after earlier blasting them with water cannon.

On Tuesday morning, fresh protests emerged in various parts of Yangon, including near the headquarters of the National League for Democracy On Tuesday morning, fresh protests emerged in various parts of Yangon, including near the headquarters of the National League for Democracy Photo: AFP / Sai Aung Main

"They fired warning shots to the sky two times, then they fired (at protesters) with rubber bullets," a resident told AFP.

At least one emergency room doctor said the military were also using live rounds, leaving a 23-year-old man and 19-year-old in critical condition in hospital.

"We believe they are actual bullets because of the wounds and their injuries," the doctor said.

The three-finger salute has become the trademark sign of protesters in Myanmar The three-finger salute has become the trademark sign of protesters in Myanmar Photo: AFP / Sai Aung Main

The father of one of the victims said his son had been shot "when he tried to use the megaphone to ask people to protest peacefully after the police used water cannon to disperse them."

"He got hit in the back... I'm very worried about him," the 56-year-old goldsmith told AFP.

In Mandalay, the country's second-biggest city, police fired tear gas to disperse protesters.

A witness, who declined to be named out of fear of the authorities, said she gave shelter to about 20 protesters, offering them water, towels and fresh face masks.

Earlier this week, the protests by hundreds of thousands appeared to have rattled the military, with junta chief General Min Aung Hlaing making a televised speech Monday to justify seizing power, citing election fraud claims.

He promised things would be different from the army's previous 49-year reign, which ended in 2011. The military also announced a curfew and a ban on protest gatherings.

But on Tuesday, fresh demonstrations erupted in Yangon, including near the NLD's headquarters, with protesters carrying placards calling for Suu Kyi's freedom and declaring "No dictatorship".

By afternoon, thousands had gathered nearby, carrying umbrellas and wearing raincoats as they faced off against police, who had water cannon trucks blocking protesters from marching.

"Of course we are worried (about a crackdown)," said protester Khin Thida Nyein, a teacher. "We only have one life but we still come out.... as we are more concerned for the future of our children."

Protesters dispersed by nightfall, with no reports of clashes.

Meanwhile, civil aviation workers and air traffic controllers have joined a civil disobedience movement, with their strike set to impact international flights wanting to pass through Myanmar's air space.

It will also hit the military government's coffers, which are set to lose overflight fees paid by airlines that could be worth up to $182,000 per day.

The NLD won national elections in November by a landslide, but the military never accepted the results.

It has announced a one-year state of emergency and promised to hold fresh elections after that.

The UN Human Rights Council said it would hold a relatively rare special session on Friday to discuss the crisis.