A sea of democracy activists flooded the streets of Hong Kong Sunday in a defiant show to the city's leaders that their movement still pulls wide public support, despite mounting violence and increasingly stark warnings from Beijing.

Tens of thousands of protesters carrying umbrellas poured across the heart of Hong Kong island defying torrential rain and a police order not to march from a park where they had gathered earlier for a rally.

Weeks of demonstrations have plunged the financial hub into crisis, with images of masked black-clad protesters engulfed by tear gas during street battles against riot police stunning a city once renowned for its stability.

Sunday's action, the largest rally in weeks which continued into the night, was billed by organisers as a return to the "peaceful" origins of the leaderless protest movement.

"It's been a long day and we're very tired, but to see so many people out in the rain marching for Hong Kong gives strength to everyone," said Danny Tam, a 28-year-old graphic designer.

The unprecedented political crisis was sparked by opposition to a plan to allow extraditions to the Chinese mainland.

But protests have since morphed into a wider call for democratic rights in the semi-autonomous city.

Anger has been sharpened among protesters by the perceived heavy-handedness of the police who have used tear gas, baton charges and rubber bullets in incidents that have pinballed across social media.

"The police are doing things that are totally unacceptable," said Yim, who like many of the protesters gave only one name.

"They are hurting citizens, they aren't protecting us."

Communist-ruled mainland China has taken an increasingly hardline tone towards the protesters, decrying the "terrorist-like" actions of a violent hardcore minority among the demonstrators.

Despite the near-nightly clashes with police, the movement has won few concessions from Beijing or the city's unelected leadership.

The spiralling violence, which last week saw protesters paralyse the city's airport, has tarnished a campaign that had taken pride in its peaceful intent and unpredictability - which demonstrators have tagged with the slogan "Be Water."

Sunday's rally was convened as a "non-violent" demonstration, according to organisers the Civil Human Rights Front.