Many wore traditional clothing and used their mother tongues for the occasion in this ethnically mixed country.
The assembly has been given the initial task of rubber-stamping the abolition of the monarchy.
But the vote was delayed while the Maoists and the other main parties settled differences about distribution of power between the president and the prime minister in an interim period.
The assembly then has two years to come up with permanent arrangements for a new constitution.
Hindu militants
Reports said King Gyanendra and Queen Komal were seen driving out of the royal palace on Tuesday afternoon, but it was not clear where they were going or for how long they would be gone.
Nepal's progress towards becoming the world's newest republic has been marred by bombs being planted in the capital for three days running this week.
One person was injured when a bomb exploded at an open-air theatre in Kathmandu on Wednesday evening.
Another went off outside the assembly venue but no-one was hurt.
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