Soldiers and police have killed hundreds in a brutal campaign against mass anti-coup protests demanding a restoration of democracy and the release of detained civilian leader Aung San Suu Kyi.

At least 107 more people -- including seven children -- were killed on Saturday, the United Nations said, as the regime staged a major show of might for Armed Forces Day -- an annual parade showcasing Myanmar's military prowess.

"It's absolutely outrageous and based on the reporting I've gotten, an awful lot of people have been killed totally unnecessarily," Biden told reporters.

European Union foreign policy chief Josep Borrell said the junta's celebration of its armed forces had been blighted by "a day of horror and of shame".

The death toll from crackdowns since the February 1 coup has climbed to at least 459, according to the Assistance Association for Political Prisoners (AAPP) monitoring group.

Saturday was by far the deadliest day of violence since the coup and AAPP said a further 13 people were killed on Sunday.

Despite the weekend violence, protesters turned out at dawn in Kachin state, while students hit the streets in Monywa in the northwest and Mawlamyin city in Mon state, according to local media.

Hundreds also paraded through the town of Plate, in Mandalay region, with banners saying: "The people will never be defeated".

Monday saw funerals for some of those killed over the weekend.

In Sagaing region, hundreds of mourners lined the street to pay tribute to 20-year-old nursing student Thinzar Hein, who was shot dead while helping rescue workers provide first aid to injured protesters.

As child casualties mount, 60 youngsters in a town in eastern Karen state staged their own protest parade accompanied by their mothers, local media reported.

In Yangon, a one-year-old girl is recovering from surgery after being shot in the eye with a rubber bullet while playing near her house on Saturday, which was also her birthday.

"She is stable, but full recovery for her eye will take time," a friend of the family told AFP.

- 'Shameful, cowardly, brutal' -

"The shameful, cowardly, brutal actions of the military and police -- who have been filmed shooting at protesters as they flee, and who have not even spared young children -- must be halted immediately," UN envoys Alice Wairimu Nderitu and Michelle Bachelet said in a joint statement.

Military-run broadcaster Myawaddy TV gave Saturday's death toll as 45 and said the crackdown was necessary because protesters had used guns and bombs against security forces.

The weekend's events brought a fresh chorus of international condemnation after weeks of so far unheeded pleas for military restraint from around the world.

There was a highly unusual joint statement from the defence chiefs of the United States, Britain, Japan and nine other countries denouncing the Myanmar military's actions.

Junta leader General Min Aung Hlaing had issued a threat to the anti-coup movement on Saturday, warning that acts of "terrorism" were unacceptable.

On Monday, the British foreign ministry advised its nationals in Myanmar to leave as soon as possible, following "a significant increase in the level of recent violence".

- Air strikes in Karen state -

An estimated 3,000 people fled through the jungle to seek safety across the border in Thailand Sunday evening following targeted aerial assaults in eastern Myanmar's Karen state, the Karen Women's Organisation civil society group said.

Thai Prime Minister Prayut Chan-o-Cha told reporters in Bangkok Monday the military was preparing for further arrivals.

According to Hsa Moo, an ethnic Karen and human rights activist, the weekend airstrikes -- the first in the state in 20 years -- killed four people and left nine injured.

"They did not have shelter... there were many children with them," Hsa Moo said.

The airstrikes targeted the Karen National Union (KNU), one of the country's largest non-state armed groups.

There are fears that the Myanmar military may launch a major operation against Karen rebels, which could force more people to flee their homes.