A peaceful mass protest in Hong Kong over an extradition Bill turned violent in the early hours today as several hundred protesters clashed with police outside the city's Parliament.

Protesters charged at metal barricades and tried to force their way into the Legislative Council building, causing police to retaliate with batons and pepper spray, after having warned protesters to clear the area.

In what organisers described as the biggest protest seen in the territory since Hong Kong was returned to the Chinese by the British in 1997, hundreds of thousands took to the streets yesterday over the unpopular extradition Bill.

Protesters hoped to pressure the government to withdraw proposed changes to the extradition law which critics charged could be used against opponents of Beijing.

The rally came ahead of the second reading of the legislation on Wednesday as the government moves to secure its quick passage with the backing of pro-establishment lawmakers.

Rally organiser, the Civil Human Rights Front, a coalition of pro-democracy groups, estimated the turnout at more than a million, but police put the figure at 240,000.

Among those braving the sweltering heat was programme officer Omana George, 47, who said the rally was all about hope and "striving for what we believe in". "People's power is very strong. We believe that if we come out and we say what we believe in, it can bring change, " Ms George said.

In 2003, some 500,000 people hit the streets to oppose government plans on national security laws, which were later shelved.

A 42-year-old protester, who gave his name only as Mr Benjamin L, said he believed many of those who publicly support the government are secretly opposed to the changes to the extradition law, "but they need Hong Kong people to come out and protest, to give them some courage to say 'no' ".

The Bill, which could be passed by the end of this month, is meant to allow Hong Kong to hand over fugitives to various jurisdictions, such as Taiwan and China.

It came about after a Hong Kong resident, Chan Tong-kai, confessed to killing his girlfriend in Taiwan last year, but he remains in the city as it does not have an extradition agreement with Taiwan. The proposed changes were floated in February and subsequently scaled back twice after widespread opposition, including from businessmen, diplomats, local and foreign business chambers, lawyers and journalists.

The massive turnout yesterday is expected to add to mounting pressure for Hong Kong leader Carrie Lam to drop the proposed changes. She has said there is no going back.

On May 30, the government said it would allow suspects or fugitives to be extradited if their offence is punishable by seven years in jail instead of three years in jail, which was already a departure from the one year stated previously.

The government has also given the assurance that there are sufficient human rights safeguards and protection of the local judiciary.

But Ms Suki Chung, 39, from Amnesty International in Hong Kong, said these safeguards are not written into the law itself. "We are actually facing the risk of being extradited any time soon by any excuse by the Chinese government."

The massive turnout meant that thousands were still gathered at Victoria Park, waiting to march, four hours after the rally began. The first batch of protesters reached the government's headquarters in Admiralty in under two hours.

Many protesters parked themselves around the gates of the complex, demanding a dialogue with the authorities. They said they would remain until Wednesday, if no dialogue was possible last night.

The rally kicked off at 2.20pm, 40 minutes earlier than scheduled as Victoria Park became too crowded.

Some of those who gathered were spotted carrying yellow umbrellas - a symbol of the 2014 Umbrella Movement that brought the city's main arteries to a halt for 79 days.

Students came, accompanied by their parents, along with the elderly armed with their walking sticks and young couples pushing prams.

All made their way slowly in 32 deg C heat, chanting "Step down, Carrie Lam" and "No China extradition, no evil law".

More than two hours after the rally began, metal barriers toppled, and the crowd, which initially spread over two lanes, spilled over to occupy all four lanes of Hennessy Road, a main thoroughfare.

The police were forced to open the eastbound lanes of Hennessy Road, kept free for emergency access, after scuffles broke out at a particularly packed section near the Canal Road flyover. At least seven people were arrested along the protest route for a number of offences, including theft, common assault and assaulting police.

The deluge of protesters led to overcrowded trains on the Island Line. Train operator MTR had to initiate crowd control measures at various stations, including at Tsim Sha Tsui and Mongkok in Kowloon.