With wooden catapults to launch petrol bombs and bows and arrows pilfered from sports departments, Hong Kong's democracy protesters are combining new tactics with medieval tech as they battle police.

Roads in the financial hub have been blockaded with bamboo lattices this week, while mini Stonehenge-like structures have been built from dug-up pavement as the southern Chinese city lurches deeper into crisis.

Universities have become the epicentre of battle, with students -- joined by other black-clad 'braves' of the frontline protest movement -- saying they have been pressed into the defence of their campuses by police threats.

As a rolling strike cripples the transport system of the famously frenetic city and fuels already intense clashes with police, hardcore protesters have bolstered their arsenal of Molotovs and bricks with an unlikely array of weapons.

Those include sports gear -- javelins and bows and arrows lifted from university storerooms, as well as tennis racquets to bat away tear-gas canisters.

Chairs and mattresses have been pulled from college dorms for use as barricades or shields against increasingly heavy barrages of police rubber bullets.

This homespun approach has also taken on a medieval edge in one of Asia's most modern cities.

Giant wooden catapults have been constructed from scratch, while caltrops -- three-pronged spikes made of plastic piping and nails -- have been laid to impede officers on foot alongside mazes of bricks to trip up police snatch squads.

Around a thousand protesters waited at Hong Kong Polytechnic University as Thursday afternoon wore on, anticipating a police charge in the hours ahead.

The campus faces the Cross Harbour tunnel, a key route between the Kowloon peninsula, which is connected by land to the Chinese mainland, and the finance centre of Hong Kong Island.

Protesters closed the link late Wednesday and had rolled a catapult into view of the tunnel in case police tried to breach the barricade.

Story continues

"If they come later we're going to load it with bricks, Molotov cocktails and flammable arrows," said a 23-year-old protester, giving the pseudonym Ah Fai.

- Flaming arrows -

The tactic fits a pattern. AFP photographs show a flame-tipped arrow being fired by a protester on Wednesday.

Meanwhile video clips circulating on social media show jubilant demonstrators celebrating as a practice round of material -- also ablaze -- hurtles through the air from a catapult.

Nearly six months on, the leaderless protest movement remains characterised by ingenuity and collective action.

But it is now also defined by increasing paranoia and violence in the face of a police force protesters accuse of brutality -- and an unyielding government.

At Hong Kong Polytechnic University on Thursday students set up a "customs" barrier to search all entrants including media.

"This is to prevent any plainclothes officers from entering," said Michael, a 23-year-old student, giving one name like most protesters.

"I don't know the effectiveness of it but it's better than nothing."

Police accused protesters of turning the Chinese University of Hong Kong, one of the city's most prestigious campuses and the scene of running battles on Tuesday night, into a "weapons factory".

"The truth speaks for itself," Hong Kong police spokesman John Tse told reporters on Thursday, accusing "rioters" of throwing petrol bombs off bridges, widespread arson attacks and firing arrows at a police patrol.

In return protesters face a police force armed with batons, rubber bullets, tear gas, water cannon and handguns -- a protester was shot on Monday at close range by a traffic cop.

Later that day a man shouting pro-Beijing slogans at protesters on a walkway was doused in flammable liquid and set alight.

He remains in a critical condition.

Hong Kong remains littered with barricades, broken glass and bricks -- collected into small castles in acts of defiance.

"Some are stacked for art," said a 17-year-old frontline protester who identified himself as Sam.

"Others we stacked higher so that the cops might run into them while they're running."