Many wore blue T-shirts bearing the slogan "Stop Civil War"
Several hundred people defied authorities to march through Yangon on Friday calling for an end to the festering conflict between Kachin ethnic minority rebels and Myanmar's army.
But the colourful rally, marking the International Day of Peace, was overshadowed by police threats to arrest organisers for going ahead with the march after being denied permission.
"As they demonstrated without official permission, they violated the law and bylaws. So we will charge them," said Yangon Police Major Myint Htwe late Friday after the rally ended.
"We will charge them not because they didn't rally peacefully, but because they violated the law."
The march, which culminated with Buddhist prayers and the building of a peace "milestone" at Inya lake, close to democracy champion Aung San Suu Kyi's house, was led by Kachins, artists and civil society groups.
It was held to draw attention to the Kachin conflict. Fighting has gripped Myanmar since June last year when a 17-year ceasefire between the government and Kachin rebels collapsed.
Organisers said authorities denied them permission to march for spurious reasons, citing the possibility of traffic build-up and "disturbances among the people".
"We were just protesting for peace, peacefully. So you can imagine what is happening here as they are arresting the people who tried to get peace here," Khin Sandar Nyunt told AFP, adding she was ready to face arrest for marching without permission, which carries a penalty of one year imprisonment and a $35 dollar fine.
The country's reformist government has agreed ceasefires with several other ethnic rebel groups as part of reforms since coming to power last year, but fighting rages unabated in Kachin State, in the nation's far north.
"We need the rule of law to get peace. When we get peace, we can get development... so we ask for rule of law first, then to build peace," said Nay Myo Zin, leader of civil society group the Myanmar Social Development Network.
A planned trip by bus-loads of Kachin activists from Yangon to the capital Naypyidaw was blocked early Friday by police, forcing protesters to join the Yangon rally instead, where many donned blue T-shirts bearing the slogan "Stop Civil War", or carrying banners and plastic doves.
Several rounds of talks over the Kachin conflict have been overshadowed by ongoing battles, while tens of thousands of people have been displaced.
The KIO recently urged the government to end its offensives in the state, alluding to the worsening refugee crisis and civilian death toll.
Aung Min, President Thein Sein's pointman on the peace negotiations, has been praised for acknowledging the need for compromise from the government side.
But in an interview this week he said Naypyidaw's influence over army units stationed in the area was limited.
"I cannot personally make the government troops withdraw. I can only assume responsibility for political and economic issues," he was reported as saying in The Irrawaddy Magazine, a Thai-based exile journal.
"The withdrawal is something that the commanders from both armies must agree upon."