"There is no force that can shake the foundation of this great nation," Xi said in Mandarin, according to an official translation broadcast through state media. "No force can stop the Chinese people and the Chinese nation forging ahead."
Xi did not specifically mention any other country by name, and emphasized that China would pursue peaceful development.
"Long live the great Communist Party of China. And long live the great Chinese People!" Xi concluded his speech, which lasted for less than 10 minutes.
Xi then reviewed the military, in which about 15,000 personnel were said to be participating. The review was also set to showcase 160 aircraft and 580 pieces of military equipment, including drones and missiles.
China ended the equipment portion of the military parade by unveiling the Dongfeng 41, a nuclear-capable missile. The Associated Press quoted analysts as saying the weapon could reach the U.S. in 30 minutes.
The capital city will also hold a gala Tuesday evening as part of the National Day celebrations.
Hong Kong's leader Carrie Lam traveled to Beijing for the 70th anniversary, as did Financial Secretary Paul Chan, amid 18 consecutive weeks of mass demonstrations in the special administrative region.
In his speech Tuesday, Xi said that the central government would "maintain long-term prosperity and stability of Hong Kong and Macao."
"Forging ahead, we must remain committed to the strategy of peaceful reunification, and 'One Country, Two Systems," Xi said. He added that China would "advance peaceful development of cross-strait relations, unite the whole country and continue to strive forward the complete unification of our country."
Separately, during a speech at a reception on Monday night, Xi stressed China's need to be unified under the Communist Party in order to overcome challenges.
"Unity is iron and steel; unity is a source of strength," Xi said, according to an English-language transcript from the Ministry of Foreign Affairs.
The Chinese leader reiterated the government's position of "One Country, Two Systems" on Hong Kong and Macao, as well as the "One-China" principle on Taiwan. Xi said, "The complete reunification of the motherland is an inevitable trend; it is what the greater national interests entail and what all Chinese people aspire for. No one and no force can ever stop it!"
Xi became China's leader in 2012, and abolished term limits last year.