Hundreds of thousands of protesters carrying umbrellas poured across the heart of Hong Kong island defying the downpour 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, billed as a return to the peaceful origins of the leaderless protest movement, drew more than 1.7 million people making it one of the largest rallies since the protests began about three months ago, according to organisers the Civil Human Rights Front.

It ended a weekend of protests that for the first time in weeks saw no major confrontations with police, as of early Monday morning.

"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.