"Democracy will prevail" read some banners as protesters sang and shouted slogans against the city's government, less than one month after the start of an official public consultation on Hong Kong's future electoral system.
"Because we are Hong Kong citizens, we must vote," said Sharon Tang, a 49-year-old administrative worker at a trading company, adding that residents of the former British colony have the intelligence to choose their future leaders.
Other protesters carried banners reading "Change does not come if not fought for", and huge posters depicting the city's current chief executive Leung Chun-ying with an egg plastered on his head.
The marchers left Victoria Park — still named after the 19th century British monarch who oversaw Hong Kong's seizure from China — in mid-afternoon and were to stage a rally in the Central financial district afterwards.
Organisers have said more than 50,000 people are expected to take part in the annual New Year's Day pro-democracy protest. There were no immediate police estimates of crowd size.
Marchers aim to let the Hong Kong government and China's ruling Communist Party know "that Hong Kong people need and want a real democracy", Johnson Yeung, convenor of rally organisers the Civil Human Rights Front, told AFP.
Hong Kong residents enjoy rights and freedoms unknown on the mainland despite China's resumption of sovereignty in 1997.
It has its own government and legal system under a semi-autonomous status, and Beijing has promised that its people will be able to vote for their next chief executive in 2017.
Under the current system, the city's leader is elected by a 1,200-strong pro-Beijing committee.
But many fear that China will control the choice of candidates to secure the election of a pro-Beijing person.
Yeung said he saw this year and the ongoing debate over the city's future democratic system as a "battle" that must be won.
"This rally will give the CCP (Chinese Communist Party) a clear message that if you don't give us real democracy, there will be direct action from the people," he said.
Mimicking the Occupy protests that broke out in 2011 in cities such as New York and London, some activists have threatened to take over the streets of Hong Kong's business district later this year to try to force officials to guarantee a fair electoral system.
A "New Year Civil Referendum" was also conducted in Victoria Park and over the internet on Wednesday, and drew about 50,000 voters by mid-afternoon.
The poll asked Hong Kongers their preferences on how candidates for the leadership post should be chosen in future.
There are frequent protests in the city of seven million, with discontent growing over sky-high housing prices, a growing wealth gap and slow progress towards full democracy.
Current chief executive Leung suffers from particularly low popularity ratings — 42 percent according to a December survey by Hong Kong University.