Hundreds of protesters at pro-democracy rallies in Hong Kong have been arrested on the grounds of illegal assembly and obstructing officials in their duty.

No less than 511 people have been detained, said the police, for their mass protests demanding greater democracy from China for the semi-autonomous Hong Kong.

Although the situation has eased in key regions, such as Charter Road and Jackson Road, warnings have been put up in areas of traffic congestion.

Tens of thousands of people took to the streets to mark the anniversary of Britain handing over the reins of Hong Kong to China on 1 July, 1997.

The organisers of the annual event said about 510,000 protesters participated in the rallies. Nonetheless, the government officials said only 92,000 took part. Independent researchers put the crowd estimate at anywhere between 150,000 and 175,000.

The Hong Kong police launched the pre-dawn crackdown on the protesters, who vowed to continue their sit-in in the city's financial district until the morning rush hour, after they failed to heed to dispersal warnings.

Most of the apprehended protesters were student demonstrators and all of them were taken to the Police College in Aberdeen on Hong Kong Island.

"The reason we are here is for universal suffrage and the future of our democracy. We are here to protest the government, not the police. You will regret this day for the rest of your lives," shouted a protester, according to the South China Morning Post.

The Chinese media, as expected, did not report the events, while posts linked to the Hong Kong protests have been removed from the social media as well.

The increasing upheaval in Hong Kong against mainland China has been a serious cause of concern for Beijing in recent years.

"In the past, it seemed like they were doing it step by step, but now, it's obvious that they cannot stand Hong Kong people. We are not obedient anymore, and are resisting more and more," a 30-year-old store manager, Kennie Chan, who participated in the protests, told Associated Press.