A pro-democracy protester has been shot by police in Hong Kong, as the city was hit by its worst unrest of the year.
It comes hours after China celebrated 70 years of Communist Party rule with a massive military parade.
It was the first such shooting in nearly four months of increasingly violent protests and threatened to strip the spotlight from China's carefully choreographed birthday party, designed to underscore its status as a global superpower.
While President Xi Jinping took salutes from some 15,000 troops in the capital, pro-democracy protesters in Hong Kong threw eggs at his portrait, with tens of thousands of people defying police orders to disperse.
Running battles raged for hours across multiple locations, with some hardcore protesters hurling rocks and petrol bombs, while police responded for the most part with tear gas, rubber bullets and water cannon.
In Tsuen Wan district, a police officer unloaded his weapon at close range into a young man after his unit was attacked by protesters armed with poles and umbrellas, videos filmed by student reporters showed.
Police said the officer feared for his life when he fired the weapon, hitting an 18-year-old man.
The wounded protester received first aid from officers before paramedics arrived and took him to hospital, police added.
Medical authorities said 31 people were admitted to hospital, two in a critical condition.
Hong Kong is seething with anger over Beijing's rule and many of the fights on Sunday were especially fierce, even by the standards of this summer's violence which has raged for 17 consecutive weeks.
In one clash several police and reporters were wounded by corrosive liquid thrown by protesters.
Burning barricades sent a pall of black smoke over the city, a regional hub for some of the world's biggest banks.
Former colonial ruler Britain, who handed Hong Kong back to China in 1997, decried the use of live ammunition as "disproportionate". Foreign Secretary Dominic Raab said that "it only risks inflaming the situation".
The violence cast a shadow over the lavish parade in Beijing where tanks, new nuclear missiles and a supersonic drone were paraded as Xi and other top officials - including Hong Kong chief executive Carrie Lam - watched from a rostrum overlooking Tiananmen Square.
The event was meant to showcase China's journey from a poor nation broken by war to the world's second largest economy.
Xi, who wore the distinctive "Mao suit", delivered a speech invoking the "Chinese dream" of national rejuvenation - his grand vision of restoring the country to perceived past glory.
"There is no force that can shake the foundation of this great nation," Xi said from the Tiananmen rostrum where Chairman Mao Zedong proclaimed the People's Republic of China on 1 October 1949.
Helicopters flew in a "70" formation as troops goosestepped across Tiananmen Square in what state media described as the country's biggest military parade, featuring 580 pieces of armament and 160 aircraft.
The People's Liberation Army brought out its newest hardware, including a nuclear-capable intercontinental ballistic missile with range enough to reach the entire United States and a launcher for a hypersonic glider.
Warplanes including the J-20 stealth fighter soared through the smog-choked skies, and state media said a high-altitude, high-speed reconnaissance drone made a public appearance for the first time.
"The party hopes that this occasion will add to its legitimacy and rally support at a time of internal and external challenges," Adam Ni, China researcher at Macquarie University in Sydney, told AFP.
But the unrest in Hong Kong also illustrated the challenges faced by China, which is also contending with US trade war negotiations.
Despite increasing levels of violence, there is still huge public support for a movement that presents the most serious threat to Beijing's rule since the 1997 handover.
In his speech, Xi said China "must adhere" to the one country, two systems policy governing Hong Kong and "maintain the long-term prosperity and stability" of the city.
The Beijing festivities continued with a pageant involving 100,000 civilians and 70 floats depicting China's greatest achievements.
A giant portrait of Mao, followed by those of past leaders and Xi, streamed across the avenue as the president and other officials waved.
Replicas of a space rocket, a homegrown passenger plane and high-speed trains were followed by smiling ethnic minorities - imagery that glosses over accusations of human rights abuses in the frontier regions of Tibet and Xinjiang.
The Communist Party has repeatedly defied the odds to remain in power for seven decades.
Under Mao, tens of millions of people died during the disastrous Great Leap Forward, and the country was plunged into violent chaos during the decade-long Cultural Revolution.
After Mao's death in 1976, the party launched the reform and opening-up policy under paramount leader Deng Xiaoping, starting decades of breakneck growth and development.
But the party retained a stranglehold on power, sending troops to end the biggest challenge to its rule in 1989 when pro-democracy protesters occupied Tiananmen Square.