(CNN) -- About 20 members of Myanmar's opposition were arrested Sunday while observing Martyrs' Day, a party spokesman said.
The opposition members were detained for half an hour and released, said National League for Democracy spokesman Nyan Win.
They were marching to the mausoleum of the father of opposition leader Aung San Suu Kyi.
Gunmen assassinated Gen. Aung Sanon on July 19, 1947, after Myanmar's independence from Britain. The day is observed annually as Martyr's Day.