The rally reflects surging discontent over Beijing's insistence that it vet candidates before a vote in 2017 for the semi-autonomous city's next leader.
The march comes after nearly 800,000 people voted in an informal referendum to demand a electoral mechanism that allows voters to nominate candidates.
The poll has irked Beijing, which branded it "illegal and invalid" despite the unexpectedly high turnout.
The city's Victoria Park, the starting point of the march that will culminate in the skyscraper-packed Central business district, was a sea of umbrellas and banners bearing slogans such as "We want real democracy" and "Civil nominations for all".
More From This Section
"Hong Kong is turning into a place with less and less freedom," Eric Wong, a 24-year-old photographer who took part in the rally said . "It is transforming into the mainland."
Organisers expect more than half a million people to join the rally, which would be a record high.
Paul Yip, a statistician at Hong Kong University, told the South China Morning Post he was leading a team of 10 to independently assess the crowd size, a topic of great politically sensitivity.
That allows residents liberties not seen on the mainland, including free speech and the right to protest.
But there are heightened fears that those freedoms are being eroded.
There has been a spate of attacks on media workers in recent months - including the stabbing of a liberal former newspaper editor - while pro-democracy media have complained of massive cyber-attacks.
Concerns increased in June when Beijing published a controversial "white paper" on Hong Kong's future that was widely seen as a warning to the city not to overstep its boundaries.