Protesters gather on a road outside the police headquarters in Hong Kong. (AFP)

Thousands of protesters converged on Hong Kong's police headquarters and blocked major roads Friday, demanding the resignation of the city's pro-Beijing leader and the release of anti-government demonstrators arrested during the territory's worst political crisis in decades.

The protest comes after the government refused to meet the demands of demonstrators who have marched in their millions to oppose a bill that would allow extraditions to the Chinese mainland.

The movement has morphed into a larger rebuke of Chief Executive Carrie Lam's administration.

Opposition groups, after putting on the biggest political rallies in Hong Kong's history, have called for the complete withdrawal of the legislation and for Lam to step down.

After converging at Hong Kong's main government complex before rush hour, hundreds of black-clad protesters -- many wearing face masks and shouting anti-government slogans -- poured onto Harcourt Road outside the parliament building, blocking the major artery completely before allowing some vehicles to trickle through.

Protesters, who have been largely leaderless during the rallies, then marched in the searing heat to the police headquarters, many chanting "release the righteous" and "shame on police thugs" -- referring to those detained during sporadic violence last week between demonstrators and the police.

Opposition groups have demanded an investigation into allegations of police brutality and the release of those detained during the clashes, in addition to Lam's ouster and a cancellation of the extradition bill.

Some protesters removed metal barricades and re-arranged them in an apparent bid to fortify their positions outside the police headquarters, as officials closed the gate to the facility's main driveway.

Many unfurled umbrellas to protect themselves from the sun, and first-aid volunteers sprayed water on others to cool them down.

"The government still hasn't responded to our demands, After so many days... they are all talking rubbish and shifting the blame on one another," protester Poyee Chan, 28, told AFP.

"So I feel we need to come out and tell them: we citizens won't accept such fake responses."

"Blossom everywhere"

The call for Friday's protest was made by the city's student unions, as well as informal organisers over social media and messaging apps like Telegram.

"Blossom everywhere," read a statement circulated Thursday in a Telegram chat group.

"There are many ways to participate. Think carefully about your own ways to show your love to Hong Kong. June 21 is not the end of the fight, there will be more in the coming days."

Protesters started to gather before rush hour at the government complex, initially settling around the legislative council building before the march towards the police headquarters.

Some organisers had also recommended a mass strike, but it was not immediately clear which business or professional groups would support such a call.

Lam has so far defied calls to step down, and while she has apologised and suspended the bill indefinitely, it has failed to quell anger.

Administrative offices at the complex were closed on Friday "due to security considerations".

Although Hong Kong was returned from British to Chinese rule in 1997, it is still administered separately under an arrangement known as "One country, two systems".

The city enjoys freedoms unseen on the mainland but many residents have been alarmed in recent years by what they feel is a tighter grip by Beijing.

Opponents of the extradition bill fear it will ensnare the people of Hong Kong in mainland China's opaque and politicised justice system, and also give Beijing a tool to target its critics based in the semi-autonomous territory.

The Chinese government had supported the extradition proposal, and accused protest organisers of colluding with Western governments. It dismissed expressions of support for the Hong Kong opposition as interference in the city -- and China's -- internal affairs.

But Beijing said after the bill's suspension that it respected and understood the decision.