Pro-democracy protestors in Hong Kong are re-occupying central Hong Kong at midnight (local time), after riot police failed to disperse the crowds with baton charges and tear gas.

The protests are some of the worst scenes of disorder since the city was handed back to China in 1997.

At least 30 people were injured in the disorder, as Hong Kong's police force disrupted protests that had shut down central Hong Kong for most of Sunday.

The protests – "Occupy Central with Love and Peace" – have been gathering momentum since China's government announced in August that the city's next chief executive will not be freely elected, but must be chosen from one of several candidates hand-picked by Beijing.

Hong Kong residents had been promised free elections as part of "one country, two systems" principle in which Hong Kong has a different political and government system to that of mainland China.

The current chief executive of Hong Kong, Leung Chun-ying, said before the violence began, that police would treat protestors firmly if they defied the law: "The police are determined to handle the situation appropriately in accordance with the law."

Despite the warning up to 80,000 protestors converged on roads around Admiralty, in Hong Kong's Central district by Sunday evening (local time).

Police in riot gear used pepper spray to try to clear the crowds, but protesters came prepared with goggles and umbrellas in a bid to protect themselves.

Police then baton-charged protesters who responded by shouting abuse and throwing missiles, in violent scenes rarely seen in Hong Kong.

A spokesperson for China's Hong Kong and Macau Affairs Office said China's central government backed the crackdown by police.

In an apparent sign China has been unnerved by the protests, the Chinese social network site Weibo blocked the phrase "Occupy Central", having earlier allowed it.

Publisher Jimmy Lai, one of the main figures behind the pro-democracy movement, said: "The more Hong Kong citizens come, the more unlikely the police can clear up the place. Even if we get beaten up, we cannot fight back. We will win this war with love and peace."