The event is the country's most important of the year as China looks to project confidence in the face of mounting challenges, including nearly four months of anti-government protests in Hong Kong and a economy-sapping trade war with the United States.
Dressed in a slate grey "Mao" suit and accompanied by his predecessors Hu Jintao and Jiang Zemin, Xi pledged peaceful development.
"No force can ever shake the status of China, or stop the Chinese people and nation from marching forward," Xi said from the Gate of Heavenly Peace, where Mao Zedong proclaimed the founding of the People's Republic of China on this day in 1949.
China must maintain lasting prosperity and stability in Hong Kong and Macao, promote the peaceful development of relations with Taiwan, and "continue to strive for the motherland's complete reunification," he added.
Xi, whose military modernisation program has rattled nerves around the region, then descended to the street and inspected row upon row of military hardware and immaculately presented troops.
Riding past in a black limousine, Xi bellowed; "Hello comrades, hard-working comrades!" The massed ranks of soldiers shouted back: "Follow the Party!
Fight to win!
Forge exemplary conduct!" Xi faces mounting challenges, notably in Hong Kong, where more large-scale protests are expected on Tuesday.