Hong Kong police shot and wounded a pro-democracy protester on Tuesday during the city's worst unrest of the year, 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 about 15,000 troops in the capital, pro-democracy protesters in semi-autonomous Hong Kong threw eggs at his portrait, with tens of thousands of people defying police orders to disperse.

Protesters say Hong Kong's freedoms are being eroded by Beijing.

Running battles raged for hours across multiple locations. Some hardcore protesters hurled 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 fired 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 shot, hitting an 18-year-old man whose condition was unknown.

The wounded protester received first aid from officers before paramedics arrived and took him to hospital, police added. 

Police said they arrested more than 180 people and officers fired six gunshots in total. 

Medical authorities said 66 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 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.

Driven by anger and grief, thousands of people came on to the streets of Hong Kong on Wednesday to denounce the shooting of a teenage student by police, an escalation of force that has intensified the standoff between protesters and authorities.

In the daytime the protesters marched through the city centre, organised sit-ins at schools and gathered at a courtroom where other demonstrators faced rioting charges. In the evening thousands more joined largely peaceful rallies across Hong Kong, denouncing police brutality. Some called for the police force to be disbanded.

Many at the demonstrations held their hands over the left side of their chests in tribute to 18-year-old Tsang Chi-kin, who was shot at point-blank range on Tuesday, with the bullet narrowly missing his heart. Tsang was in hospital in stable but critical condition after surgery to remove the bullet.

The shooting shocked many in the city; despite copious use of teargas, water cannon, beanbags and other less lethal forms of violence over nearly four months of protests, officers had previously only fired their guns in warning.