The four-day standoff between demonstrators and police at Hong Kong Polytechnic University (PolyU) has also rippled overseas, with the UN's human rights office urging a peaceful resolution, while the US senate passed new legislation supporting protesters' demands.

The focus of nearly six months of increasingly savage anti-China protests has shifted to the PolyU campus, a stone's throw from the city's harbour, where hardcore protesters have held off riot police with Molotov cocktails, bricks and arrows.

Seven hundred people have been arrested across "the whole PolyU incident," chief superintendent Ricky Ho told reporetrs late Wednesday giving updated figures.

Around 300 protesters aged under-18 in the campus also had their details taken before being "let go", he said.

Among the arrested were two protesters who were held on Wednesday as they tried to emerge from a sewer around half a kilometre outside the campus - a sign of the desperate measures being taken to break the police cordon.

The protest movement began over a now-shelved bill to allow extraditions to China, which revived fears that Beijing was slicing into the city's freedoms.

Millions of angry citizens have hit the streets in a movement that snowballed into wider calls for free elections and an inquiry into alleged police brutality, demands that Hong Kong's Beijing-appointed leaders have rebuffed.

The city's reputation for prosperity and stability has been smashed by the unrest, tipping the economy into recession and forcing a slew of major sporting and entertainment events to be pulled.

Organisers on Wednesday postponed next week's Hong Kong Open, one of Asia's oldest golf tournaments, because of the crisis.

Protesters at PolyU said around 50 of their number remained after hundreds had fled deteriorating conditions and officials warned that police may fire live rounds.

Exhausted bands of youths continued to wander the debris-strewn campus, preparing Molotov cocktails while others slept on a gym floor.

A large "SOS" sign was laid out across an open courtyard, apparently intended to be viewed from above.

A 20-year-old protester who gave his name as Ken said food and water were running low.

"We will die because they don't let me out. Some people want to get out but they cannot. So we left this SOS," he told AFP.

The holdouts included around 20 of the university's students, a PolyU official said.

Hong Kong's chief executive Carrie Lam called Tuesday for protesters to surrender, adding that those over 18 would face rioting charges, but minors would not be arrested.

'Never give up'

"I don’t ever give up. Yes, I will fight until the end," said a 15-year-old holdout armed with bow and arrow who identified himself as William.

"But... it’s very dangerous, because when you use the bow, the police must shoot you, with some unknown bullets. Maybe real bullets."

Others were medically evacuated throughout the day.

The UN human rights office was watching the university situation with "deepening concern", spokesman Rupert Colville said in Geneva.

Colville acknowledged protesters' "deep-seated grievances" but condemned the "extreme violence" of some demonstrators and called on authorities to seek a peaceful resolution.

A "Blossom Everywhere" strategy of disruptive vandalism lasting more than a week has brought much of the city's transport network to a standstill and shuttered schools.

Some of the busiest subway lines were disrupted yet again on Wednesday, with huge queues of commuters in the morning as protesters reportedly blocked train doors from closing.

Police said 213 people were charged in court on Wednesday with rioting for one of the diversionary actions earlier this week. The charge carries a maximum of ten years in prison.

But online protest appeals again brought citizens to the streets in some places Wednesday.

US pressure

The city's education bureau reopened primary and secondary schools on Wednesday after a suspension that began last week. Kindergartens remained closed.

China has repeatedly condemned protesters as violent criminals and rejected any foreign criticism.

But the US Senate on Tuesday unanimously adopted new legislation threatening to revoke the favourable trade status that the semi-autonomous Chinese territory enjoys with the United States if its freedoms are quashed.

Lawmakers also approved a measure that would ban sales of tear gas, rubber bullets and other equipment used by Hong Kong security forces.

Beijing shot back on Wednesday, warning of "strong countermeasures" if the bill becomes law, and summoning acting US charge d'affaires William Klein to lodge a protest.

Hong Kong on Sunday holds district elections to its legislature, half of which is directly elected, the other half stacked with pro-Beijing lawmakers.

But Chief Secretary Matthew Cheung warned Wednesday that further violence could "ruin" the chances of holding the polls, in which China-aligned candidates are expected to take a drubbing.