The closed-door summit, which takes place once every five years, determines who rules China and the country's direction for the next term.
Mr Xi, who became the leader in 2012, has been consolidating power and is expected to remain as party chief.
The congress, which also decides on a roadmap for China for the next five years, is expected to finish next week.
Shortly after the congress ends, the party is expected to unveil the new members of China's top decision-making body, the Politburo Standing Committee, who will steer the country.
In his speech, Mr Xi listed China's recent achievements, saying that "socialism with Chinese characteristics in this new era" meant China had "become a great power in the world", and added that the country should not copy foreign political systems.
He also:
- Briefly described a two-stage plan for China's "socialist modernisation", to be achieved by 2050
- Warned against separatism - in an apparent reference to movements in Xinjiang, Tibet and Hong Kong - and reiterated the government's principle that Taiwan is part of China
- Said China "would not close its doors to the world" and promised further economic reform, including lowering barriers for foreign investors
'Surging ideological confidence'
Xi Jinping is a much more assertive leader than his predecessors. In a long and confident speech, he looked back on his first five years in office, saying the party had achieved miracles and China's international standing had grown.
But the most striking thing in his mission statement was ideological confidence. Recently Party media have talked of crisis and chaos in western democracies compared to strength and unity in China.
Today Xi Jinping said he would not copy foreign political systems and that the communist party must oppose anything that would undermine its leadership of China.
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