Myanmar’s first freely elected Parliament after half a century of military rule opened on Monday, a symbolic but critical milestone in the country’s fragile transition to democracy, and a moment long awaited by Daw Aung San Suu Kyi, the leader of the nation’s democracy movement.

Ms. Aung San Suu Kyi entered the parliamentary chambers in Naypyidaw, the capital, through a side door. The chamber was swathed in orange — the color of her National League for Democracy Party, which overwhelmingly won a landmark election on Nov. 8. The military, as part of a complex political transition that has unfolded since 2010, retains 25 percent of the seats in both houses; its members wore green uniforms.

At least 110 of the party’s 390 members in the new Parliament are, like Ms. Aung San Suu Kyi, former political prisoners. They were formally installed on Monday following an unusually jubilant celebration on Friday, with karaoke singing and dancing, to mark the end of the military-led Parliament.

“The honeymoon period will be brief,” U Aung Zaw, an influential journalist who returned to Myanmar in 2012 after 24 years in exile, wrote on Monday on the website of his publication, The Irrawaddy. “All the hard work lies ahead.”