By Agence France-Presse
Lum Zawng, Nang Pu and Zau Jet were sentenced Friday to six months in prison and fined about $320 each for defaming the military after helping organise a demonstration in Kachin state's capital Myitkyina in April.
They were highlighting the plight of thousands of civilians trapped by the fighting between the military and ethnic Kachin insurgents in one of the world's longest-running civil wars.
On Tuesday demonstrators, many with faces painted in Kachin's red and green colours, held banners aloft and chanted slogans against the activists' imprisonment and the judiciary.
"We demand the authorities release our people immediately," protest leader Naw Aung told AFP after the march, which wound its way to the city's main stadium.
The organisers said some 4,000 people took part in the demonstration, including many languishing in around 30 displacement camps around the city.
Police gave AFP a more conservative estimate of 1,700 participants, although acknowledged that more people joined later.
The EU and rights groups have urged Myanmar's government to quash the convictions and release all three activists.
The United Nations special rapporteur on human rights in Myanmar, Yanghee Lee, described the jail terms given to the activists as "wholly unacceptable".
"This is yet another case in which the Myanmar government is failing to uphold human rights and democratic principles, and is using an archaic law that is contrary to accepted human rights principles as a weapon against human rights defenders," she said.
Rebels have clashed with the powerful military for more than six decades in restive Kachin state over autonomy, ethnic identity, drugs, jade and other natural resources.
Fighting surged dramatically this year, forcing thousands to flee to camps in remote parts of the state with inadequate access to aid.
More than 106,000 people are still living in camps across Kachin and Shan states, their plight recently overshadowed by the separate Rohingya crisis in western Rakhine state.