Thousands of pro-democracy activists have clashed with police in Hong Kong after they tried to encircle government headquarters in the city overnight.

On Sunday evening, protest leaders at the main activists' site in the Admiralty neighbourhood announced they would escalate their campaign.

Chaos erupted as the demonstrators, wearing helmets and wielding umbrellas, charged police after officers warned them to retreat.

Riot police used batons and pepper spray to push them back.

Local media reported that at least 45 people were arrested and police said 11 officers had been injured.

Many government offices and shops remained shut by mid-morning, but the situation is said to now be calm.

Police had cleared the area more than a month ago during some of the most violent scenes since the demonstrations began in late September.

The democracy movement represents one of the biggest threats for China's Communist Party leadership since its bloody 1989 crackdown on pro-democracy student protests in and around Tiananmen Square in Beijing.

The flare-up comes after four nights of clashes in the working-class district of Mong Kok, across the harbour from Admiralty.

The Hong Kong rallies drew more than 100,000 on to the streets at their peak, but numbers have since dwindled to a few hundred.