The controversial electoral roadmap, which lays out how Hong Kong's next leader should be chosen, goes for debate at the legislature on Wednesday and will be voted on by the end of the week.
It is the culmination of a fraught chapter which saw tens of thousands of pro-democracy protesters bring parts of the city to a standstill late last year.
Those rallies were sparked by a ruling from Beijing that candidates in the city's first ever public vote for its leader in 2017 must be vetted.
Currently the chief executive is elected by a 1,200-strong pro-Beijing committee.
More than 1,000 people set off from the city's Victoria Park today afternoon, heading towards the legislative council building where they are due to rally into the evening.
With temperatures soaring to 30 degrees Celsius (86 degrees Fahrenheit) and humidity at almost 80 per cent, numbers were well below organisers' hopes as the march began -- they had said they expected 50,000 to join.
Sunday's gathering is the first of a series of rallies which activists say will take place each day until lawmakers vote on the bill.
"Vetoing (the bill) is not something to be happy about," said Labour Party lawmaker Lee Cheuk Yan, addressing the crowd Sunday.
"It shows Hong Kong people have the courage to veto the rotten proposal, but it doesn't mean we have won."
Latest figures from one joint university poll showed those against the reform package taking the lead for the first time with 43 per cent, versus 41.7 in support.
"People may feel exhausted and directionless, but... We need to fight on to get back our Hong Kong, and our basic political rights," added Johnson Yeung of Civil Human Rights Front, which organised the march.
