China celebrated seven decades of Communist Party rule on Tuesday with a massive display of military power in central Beijing, as President Xi Jinping pledged peaceful development but said the armed forces would defend China's sovereignty.

The event is the country's most important of the year as China looks to project an image of 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.

President Xi Jinping, dressed in a slate grey "Mao" suit and accompanied by his predecessors Hu Jintao and Jiang Zemin, said China will pursue a mutually beneficial strategy of opening up.

The country's military should resolutely safeguard China's sovereignty, security, and development interests, and firmly uphold world peace, Xi said in comments carried live on state television.

"No force can ever shake the status of China, or stop the Chinese people and nation from marching forward," Xi said fom 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 self-ruled Taiwan and "continue to strive for the motherland's complete reunification", he added.

Xi, whose military modernisation programme 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 remains broadly popular in China for his aggressive campaign against corruption and for propelling what is now the world's second-biggest economy to the forefront of global politics.

But the Communist Party remains nervous about its grip on power and international standing.

The capital has been locked down for the parade. Police have told residents whose houses look onto the parade route not to look out their windows.

There will be a civilian parade too, of students, model workers, ethnic minorities and even a few foreigners, walking alongside or travelling in floats celebrating China's achievements, officials said last week.

Once the show is over, 70,000 doves will be released to symbolise peace, according to state media.

In the evening, fireworks will light up Beijing.

Xi faces mounting challenges, notably in Hong Kong, where more large-scale protests are expected on Tuesday.

Police there have warned of "very serious violent attack".

Hong Kong went into lockdown on Tuesday with barricades in the city centre, shuttered stores and a heavy riot police presence. Embattled Hong Kong leader Carrie Lam is in Beijing for the anniversary celebrations.

Another challenge is Chinese-claimed Taiwan, a free-wheeling democracy with little interest in being run by Beijing and which holds presidential elections in January.

There are also restive minorities in Tibet and heavily Muslim Xinjiang, where China has faced international opprobrium for detaining up to one million ethnic Uighurs in what China calls a de-radicalisation scheme.