Police were out in force early in the main city of Yangon and elsewhere, deployed at usual protest sites and detaining people as they congregated, witnesses said. Several media workers were detained.
But people still gathered, some for a march by ethnic minorities in Yangon, their numbers building through the day.
Crowds chanted and sang, then melted into side streets as police advanced, firing tear gas, setting off stun grenades and firing guns into the air, witnesses said.
Similar scenes played out in the second city of Mandalay, and several other towns including Dawei in the south, witnesses and media reported.