The 75-year-old faces a slew of charges since being overthrown by the army in a February 1 coup.
At least six children between the ages of 10 and 16 were among those killed on Sunday.
"We just can't handle another five decades [of instability]."
Five of the 12 were shot dead when police opened fire on a sit-in protest.
At least 18 dead after Myanmar police open fire on protesters
Huge crowds are denouncing a military coup.
Three people were injured.
More than 160 people have been arrested since the military seized power on Monday.
"We want the military coup to fail."