While the new leadership lineup will not officially be known until later into the weeklong party convention that opened Thursday, Xi has been widely mentioned as successor to President Hu Jintao, and Li is likely to take over from Premier Wen Jiabao.
Their appearances in the panel discussions came after Hu's keynote address to some 3,000 delegates attending the Communist Party's 18th congress.
The official Xinhua News Agency quoted Xi as saying the meeting's lengthy theme defined the path and goal of the party that has ruled the country since 1949. The theme stresses "socialism with Chinese characteristics" and calls for implementing policy reforms and opening up for building a prosperous society.
Xi said the theme is based on the full comprehension of current world conditions, the nation and the Party, and is closely interlinked with the overall plan for promoting economic, political, cultural, social, and ecological progress.
Li said China should lose no time in deepening reform in key sectors, accelerate improvement of the socialist market economy and facilitate the change of growth model for building a moderately prosperous society, while deepening reform and opening up in an all-round way.
In his address, Hu touched on a number of issues confronting China.
He said among important achievements under his leadership in the past 10 years was introducing a scientific outlook on development, which seeks new scientific answers to major questions and following the path of socialism with Chinese characteristics.
Hu called for doubling China's 2010 gross domestic product and per capita income for both urban and rural residents by 2020 to make China's development much more balanced, coordinated and sustainable.
"The underlying issue we face in economic structural reform is how to strike a balance between the role of the government and that of the market, and we should follow more closely the rules of the market and better play the role of the government," he said.
With demand for China's exports slowing due to the global crisis, Hu called for firmly maintaining "the strategic focus of boosting domestic demand" and speeding up "the establishment of a long-term mechanism for increasing consumer demand."
On political reforms, Hu called for drawing on the political achievements of other societies but stressed "we will never copy a Western political system."
On foreign affairs, Hu said China will follow the path of peaceful development but added, "We are firm in our resolve to uphold China's sovereignty, security and development interests and will never yield to any outside pressure."
Hu dwelt on the problem of corruption facing the country, warning, "If we fail to handle this issue well, it could prove fatal to the party, and even cause the collapse of the party and the fall of the state."
Hu also called for building a "strong national defense and powerful armed forces that are commensurate with China's international standing and meet the needs of its security."