Mr Xi led the new Politburo Standing Committee out onto the stage at the Great Hall of the People in Beijing, signalling his elevation to the top of China’s ruling Communist Party.
He replaces Hu Jintao, under whose administration China has seen a decade of extraordinary growth.
The move marks the official passing of power from one generation to the next.
Mr Xi was followed by Li Keqiang, the man set to succeed Premier Wen Jiabao, and five other men – meaning that the size of the Standing Committee had been reduced from nine to seven.
Those five, in order of seniority, were Vice-Premier Zhang Dejiang, Shanghai party boss Yu Zhengsheng, propaganda chief Liu Yunshan, Vice-Premier Wang Qishan and Tianjin party boss Zhang Gaoli.
The new leaders had great responsibilities, Mr Xi said, but their mission was to be united, and to lead the party and the people to make the Chinese nation stronger and more powerful.
“The people’s desire for a better life is what we shall fight for,” he said.
Corruption had to be addressed, he said, and better party discipline was needed. But the new leaders would strive to deliver a good performance in the eyes of the people and the eyes of history.
The new Standing Committee was endorsed in a vote early on Thursday by the new party Central Committee, but in reality the decisions had been made in advance.
The new leaders will gradually take over in the next few months, with Hu Jintao’s presidency formally coming to an end at the annual parliament session in March 2013.
Mr Xi has also been named chairman of the Central Military Commission, a Xinhua news agency report said, ending uncertainty over whether that post would be transferred from Hu Jintao immediately.
Mr Hu’s predecessor, Jiang Zemin, held on to the post for two years after he stood down from the party leadership.
Mr Xi, a former Shanghai party chief, was appointed to the politburo in 2007.
A “princeling” – a relative of one of China’s revolutionary elders – he has spent almost four decades in the Communist Party, serving in top posts in both Fujian and Zhejiang provinces, as well as Shanghai.
The 59-year-old is said to be a protégé of Jiang Zemin, while Li Keqiang is said to have been Mr Hu’s preferred successor. (BBC)