The Nepali Congress opened its 15th general convention in Kathmandu on Sunday, drawing more than 7,700 elected delegates to the Khula Manch grounds and leaders from rival parties who used the stage to press for generational change and cooperation among the major parties.

Party President Gagan Thapa inaugurated the three-day gathering, his first full convention since winning the presidency at a special convention in January, laying wreaths at portraits of former party presidents, lighting a ceremonial lamp and watering a sapling. Former prime ministers Pushpa Kamal Dahal and Baburam Bhattarai and the CPN-UML general secretary, Shankar Pokhrel, joined in paying tribute.

Mr. Bhattarai delivered the day’s sharpest message, telling leaders of his own generation to step aside after last year’s Gen Z-led protests. “My message to leaders of my generation is: let us learn from the past. We must now trust the new generation,” he said. “The Congress has gone through a generational transition, but it has not been able to maintain unity within the party.”

Nepal should not blame outside forces for the protests without asking why young people had lost faith in politics, he said. “We need to ask why we failed to convince young people after establishing federal democracy.” He pointed to weak economic growth, the large numbers of Nepalis working abroad and the country’s dependence on remittances as signs the system had failed to deliver opportunity. He urged parties to protect the constitution and its pillars of federalism, democracy and inclusion, backed a directly elected executive and said the Rastriya Swatantra Party government of Prime Minister Rabi Lamichhane should be allowed to complete its five-year term.

Mr. Pokhrel offered an endorsement of sorts to the Congress president. “Through the 15th General Convention, you will establish yourself as an acceptable president,” he said, recalling Mr. Thapa’s years as a student leader in the republican movement. The UML’s chairman, KP Sharma Oli, skipped the event and sent Mr. Pokhrel in his place, according to UML officials.

Sharat Singh Bhandari, co-chair of the Janata Samajbadi Party-Nepal, said Nepalis would not accept any form of “controlled democracy” and called for national unity and constitutional amendments where needed. Rajendra Lingden, chairman of the Rastriya Prajatantra Party, called for national consensus and said political attacks had shifted online, with social media replacing the physical violence of the past. Resham Chaudhary, a founding leader of the Nagarik Unmukti Party, said constitutional guarantees for communities such as the Tharus remained unmet: “Rights were written into the Constitution, but they were not delivered.” Aaren Rai, general secretary of the Shram Sanskriti Party, attending in place of its chairman, Harka Sampang, said Congress had been built through decades of struggle and urged it to worry less about holding power and more about delivering.

Not every guest of note appeared. CK Raut, chairman of the Janamat Party, stayed away because his party is holding its own second general convention in Birgunj. Mr. Dahal, coordinator of the Nepali Communist Party, and Mr. Lamichhane attended or were expected, alongside Mr. Lingden.

Delegates streamed in through the morning from hotels around Gaushala, Koteshwar, Lazimpat, Thamel and other neighborhoods, with parking arranged at the underground Dharahara facility, the old Bus Park, Bhrikutimandap and the City Hall; buses from outside Kathmandu were given one lane between Maitighar and Bhadrakali. The opening ceremony began at 2 p.m.

The convention is the party’s first full-scale leadership election under Mr. Thapa and is expected to set its direction into the next electoral cycle. Closed-door sessions follow on Monday and Tuesday, with voting for new leaders on Wednesday and Thursday.

The gathering also carries structural changes adopted at the January special convention: guaranteed representation for members under 35, at least 21 young people on the central working committee, a minimum one-third share for women and more seats for marginalized and minority communities. The proposed new central working committee would have 232 members, and the party is weighing additional vice presidents and general secretaries.

Races are already forming. Abdul Sattar, a central committee member, declared for joint general secretary from the Muslim cluster; Deepak Giri announced for vice president; and the youth leader Shankar Tiwari entered the general secretary race, saying he wanted to strengthen Mr. Thapa’s leadership while providing internal checks on him.

Pushpa Bhusal, a Congress vice president, told the opening session the convention should be more than a division of posts. “It should be a forum for Congress leaders and activists, as well as everyone who believes in democracy and the Congress, to engage in self-reflection,” she said. Krishna Sabuj Baniya, the party’s Kathmandu district president, said the gathering would fulfill the mandate of the special convention and asked delegates to address the party’s shortcomings positively. Voting begins Wednesday.