Hong Kong is a former British colony handed back to China in 1997 under a model known as "one country, two systems".

It has its own judiciary and a separate legal system from mainland China.

Those rights include freedom of assembly and freedom of speech. But those freedoms - the Basic Law - expire in 2047 and it is not clear what Hong Kong's status will be at that point.

For many of those on the streets, that is precisely what is at stake.

Timeline of Hong Kong protests

How is Hong Kong run?

Meanwhile, police in Hong Kong have been speaking to reporters:

They say they heard from a source that some protesters changed their clothes and wore medics vests or press vests, in order to escape.

Last night police arrested 51 so-called medics and reporters and some of them are not able to prove their identities, according to police.

New comments have just come in from China's foreign ministry. Remember Hong Kong is a part of China and these protests are, in part, about the fear that the special freedoms Hong Kong enjoys as a former British colony are being eroded.

Foreign ministry spokesperson Geng Shuang said at a daily news press conference: "No one should underestimate China's will to safeguard its sovereignty and Hong Kong's stability."

The action is not just at the university.

Protesters have also gathered in various other locations to either distract the police or pressure them into letting the PolyU protesters go.

At one site in the busy shopping district of Tsim Sha Tsui, some have formed long human chains to pass supplies and projectile weapons such as bricks. Martin Yip of BBC News Chinese describes the scene.

Some more insight - though nothing is yet confirmed - for the smoke at the scene. The Guardian's Beijing bureau chief Lily Kuo has just tweeted this video saying she heard several explosions at PolyU.

The key question for many has been: are they students or protesters unrelated to the university? And how many are left?

In terms of numbers, local media reports quote Derek Liu Kin-kwan, president of the PolyU student union, as saying that about 600 people are still inside. A lawmaker, Ted Hui, said it's up to 1,000. Mr Liu says 60-70% of those left are students of the university.

But that's at odds with earlier university official statements which said the campus had become "occupied by activists". A Hong Kong lawmaker even said he believed some of those inside were secondary school students. Police have been labelling those left inside a "radical" hardcore.

So, it's not entirely clear - but we know they are young and likely running out of supplies.

BBC producer Orsolya Szoboszlay who is near the university has just sent in this picture of thick black smoke coming from the main entrance. It's unclear what's causing it.

The BBC's Robin Brant was outside PolyU when a group of protesters attempted to leave the university - and confrontations amid tear gas ensued.

After Sunday, when protesters barricaded themselves inside the university, they were set a 2200 deadline to leave PolyU by police. They missed that deadline and elite police launched a dawn raid but had to retreat.

The clashes erupting now come after a relative lull in the action - and in that time images emerged of exhausted protesters.

These latest clashes have come as these protesters tried to leave the university through the main entrance.

Image source, Photo by Laurel Chor/Getty Images

Image source, Photo by Laurel Chor/Getty Images

Hong Kong Free press news website tweets that Red Cross volunteers were allowed by police to enter the PolyU campus. This comes after multiple reports of injuries inside.

The BBC's Gabriel Gatehouse in Hong Kong says that the ground outside PolyU is littered with rubber bullets.

Earlier we saw them fired at protesters as they attempted to leave the campus.

The BBC's Robin Brant has posted a series of images of police detaining protesters at the PolyU.

It's an insight into the chaotic nature of the scene - but also how control of key roads can shift between protesters and police.

Our correspondent added that the police were trying to regain control over the main road flyover in the area.

In the past few hours, we have seen several attempts by protesters to leave the campus.

A BBC team stationed outside PolyU captured the moment one group tried to escape.

For their part, police say they have asked protesters to leave in an "orderly fashion" and say they were "charged" by demonstrators.

Image source, Getty Images

For months, Hong Kong has been rocked by anti-government protests and the situation shows no sign of calming down.

It's easy to lose sight of what it's all about, so here's all the background you need to know in 100 or 500 words - you can read each individually or in turn.

Journalist Laurel Chor tweets that she saw "multiple subdued protesters getting dragged along concrete" and that several of them "had bloodied faces".

Monday morning was relatively calm compared with the ferocity of the previous night where intense clashes saw police try and fail to storm the campus. But as the day wore on the confrontations intensified. So far today:

• Protesters attempted to leave the campus early in the morning only to retreat in the face of police tear gas
• Elsewhere in the city, protesters attempted to divert police by setting up roadblocks. Many were arrested in large numbers
• In the heart of the city's financial district at lunchtime office workers came out to protest, a pattern seen during last week's protests too
• Police urged protesters to leave "in an orderly manner" and blamed "masked radicals" inside the campus for the violence
• The government announced that schools across the territory will remain closed for another day

The latest images from the Polytechnic University (PolyU) campus, where riot police have been detaining protesters trying to leave the area, go some way to convey the chaos and destruction that has unfolded over the past 24 hours.

Image source, Reuters

Image source, Reuters

Image source, AFP/Getty Images

Image source, Reuters

Welcome to the BBC’s live coverage of the ongoing stand-off between the hundreds of protesters inside Hong Kong’s Polytechnic University and police surrounding the campus.

Ever since intense clashes erupted at the campus overnight, police and protesters across the city have been engaged in a tense game of cat and mouse.

We’ll be bringing you analysis and all the latest updates from our reporters on the ground.

Image source, Getty Images