Protesters armed with little more than parasols and determination have brought central Hong Kong to a standstill with their demands for full democracy, in a movement that has been dubbed the "umbrella revolution".

The umbrella is fast emerging as the symbol of the demonstrations that since Sunday have paralysed the Asian financial hub -- a quintessential image in a city known for its downpours.

The demonstrations that have brought thousands onto the streets turned violent Sunday evening as police used tear gas and pepper spray against the swelling crowds.

The protesters -- mostly university students and high school pupils who last week boycotted classes in an attempt to pressure Beijing -- were ill-equipped to deal with the sudden violent turn.

They wrapped their eyes in clingfilm or donned goggles, wore paper face masks and cowered behind umbrellas to try to protect themselves from the tear gas and pepper spray.

"The umbrella is probably the most striking symbol of this Hong Kong protest. Our demonstrations used to be so peaceful, even pepper spray was very out of the ordinary," said Claudia Mo, a pro-democracy lawmaker.

"Now that pepper spray has become so common, we're having to use umbrellas against it."

"The police have very high-quality shields -- we just have our umbrellas."

The phrase "umbrella revolution" was trending on social media Monday, and was also seen scrawled on a banner flung across a pile of upturned barricades and discarded umbrellas than blocked the entrance to a metro station.

- Festival atmosphere -

After the tear gas had blown away in the early hours of Monday, a festival atmosphere descended on the streets.

In the busy shopping district of Causeway Bay, more than a thousand demonstrators began chanting soft rock songs by the 1980s Hong Kong band "Beyond" after a resident in a tower block started playing the song over loudspeakers.

The band's melancholy ballad "Boundless Oceans, Vast Skies" has often been sung by the city's various protest movements who embrace the chorus lyrics: "Still free and independent/Forever singing my own song out loud".

"Everybody knows the lyrics. I'm in my 40s but look at these kids, they know it too," said one man at the protest who gave his surname as Bun.

By Monday morning calm had largely returned to the city's streets, although the usual throng of smartly-dressed commuters was replaced by a sea of dishevelled protesters who had spent the night camped on the streets -- under umbrellas.

Demonstrators sorted rubbish from the previous day into piles for recycling and distributed food and water amongst themselves.

The scenes were reminiscent of the 2011 Occupy Wall Street movement where demonstrators set up generator-powered mobile phone recharging stations, a volunteer-run food tent and even a library.

"The cops, they are the ones who did violent things, attacking people without weapons. It really stimulates our emotions, so as normal people we know we need to do something," said bank worker Maple Leung, 27, who was out Monday distributing food and water to protesters.

Amid the chaos that has taken the city by surprise, the Chinese flag atop the Admiralty Centre near the government headquarters was raised upside down Monday morning. As a team climbed onto the roof to correct the mistake, the crowd of demonstrators booed.

Protesters have vowed to remain on the streets until Beijing meets their demands for unfettered elections.

"I'm staying until the end, until we get what we want to get, which is true democracy," said 18-year-old high school student Michael Wan.