In a statement to mark the anniversary, the party reiterated its call for the immediate release of all political prisoners, including Suu Kyi, who has been under house arrest for most of the last 19 years, and her deputy Tin Oo.
It also demanded the freedom of Buddhist monks and ethnic leaders arrested by the military government.
"An indelible black stain will be tainted in the political history of Burma by the omission of the authorities to perform according to the laws enacted by themselves," it said, referring to the country by its former name.
Shortly before the ceremony, a small group of NLD members shouted: "May Daw Aung San Suu Kyi be free. May all political prisoners be free."
|Win Tin was freed after 19 years
in jail [AFP]
Win Tin, who spent 19 years behind bars, was one of seven party members who were released on Tuesday as the government granted an amnesty to 9,002 prisoners around the country.
"While I was in prison I always kept three main things in mind - to support the NLD, to support the People's Parliament and to support the leadership of Aung San Suu Kyi. That is how I survived," he said during the event.
However, one of those freed, Win Htein, a close aide to Suu Kyi, was rearrested jst 17 hours later. No reason was given for returning him to detention.
Aung Shwe, the party chairman, said the rearrest was "very unfortunate".
The party was founded in 1988 after a failed pro-democracy uprising, and has faced nearly constant harassment from the ruling generals since then. It won the most seats in a general election held in 1990, but the generals refused to recognise the result.
The 20th anniversary celebrations came a year after a bloody crackdown on protests against military rule in which 31 people were killed and thousands more were arrested.