Vast crowds of pro-democracy protesters have thronged Hong Kong's streets in a display of support for the movement as it marks six months with organisers warning the city's pro-Beijing leaders they had a "last chance" to end the political crisis.

Hundreds of thousands snaked their way through the financial hub's main island in the biggest turnout for months.

Organisers estimated the turnout at 800,000 while police said it was 183,000.

As night fell the crowds switched on their mobile phone torches, creating a glittering carpet of lights that stretched far into the distance, their chants bouncing off the towering skyscrapers above.

The rally, which received rare police permission, came two weeks after pro-establishment parties got a drubbing in local elections, shattering government claims that a "silent majority" opposed the protests.

Many of the black-clad attendees voiced anger and frustration that chief executive Carrie Lam and Beijing have ruled out any further concessions despite the landslide election defeat.

Hong Kong has been battered by increasingly violent demonstrations in the starkest challenge the city has presented to Beijing since its 1997 handover from Britain.

Millions have marched in protests fuelled by years of growing fears that authoritarian China is stamping out the city's liberties.

The last fortnight has seen a marked drop in violence and vandalism after the landslide win by pro-democracy candidates.

Police took the unusual step of allowing the Civil Human Rights Front (CHRF) to hold today's march - the first time the group has been granted permission since mid-August - but they warned they would have zero tolerance for violence.

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The movement's demands include an independent inquiry into the police's handling of the protests, an amnesty for those arrested, and fully free elections.

"Our rally today is to gather everyone in Hong Kong to defend our city," the CHRF said in a statement.

Earlier, the group's leader Jimmy Sham said the march was Ms Lam's "last chance" to listen to the people.

The protests were initially sparked by a now-abandoned attempt to allow extraditions to the mainland but have since morphed into a popular revolt against Beijing's rule.

The CHRF, which advocates non-violence, has been the main umbrella group behind record-breaking rallies earlier in the summer that saw huge crowds regularly march in searing heat.

Authorities have repeatedly banned major rallies in recent months citing the risk of violence from hardcore protesters.

Large crowds have simply ignored the bans, sparking near-weekly tear gas and petrol bomb clashes that have upended Hong Kong's reputation for stability and helped tip the city into recession.

Hours before the march was due to start, police displayed weapons, including a pistol and knives, they said had been found during overnight raids where eleven people were arrested.

Tomorrow marks the six-month anniversary of the protests during which some 6,000 people have been arrested and hundreds injured, including police.

Online forums used to organise the movement's more radical wing have vowed to target tomorrow morning's commute if there is no response from Ms Lam.

But there is little sign she is willing to budge, leading to fears the lull in street clashes will be temporary.

In a statement released ahead of the march, Hong Kong's government said it had "learned its lesson and will humbly listen to and accept criticism." But it announced no concrete measures or concessions.

Since the local elections the city's chief executive has remained steadfast in her opposition to further concessions and Beijing has stuck by her even as she languishes with record low approval ratings.

The police force's reputation has also taken a hammering.

Police have defended their handling of the protests and say their force has matched rising violence from hardcore protesters.