Pro-democracy campaigners took to the streets of Hong Kong Sunday, but in far smaller numbers than expected, before a vote on a political reform package that has divided the city and sparked mass protests.

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.

Pro-democracy lawmakers in the semi-autonomous Chinese city have vowed to vote down the election package, which sticks to Beijing's ruling.

Currently the chief executive is elected by a 1,200-strong pro-Beijing committee.

Around 3,500 people joined the march through central Hong Kong from Victoria Park to the legislative council, organisers said.

With temperatures soaring to 30 degrees Celsius (86 degrees Fahrenheit) and humidity at almost 80 percent, numbers were well below organisers' hopes -- they had said they expected 50,000 to join.

Civic party leader Alan Leong said people had stayed at home because they trusted pro-democracy lawmakers to vote down the government's proposal.

"Hong Kong people trust the pan-democratic legislators will definitely veto the government proposal," Leong told AFP.

"Why spend a hot Sunday under the sun?"

Others said the low numbers were due to protest fatigue.

"We have had numerous protests in the past and some HongKongers may feel powerless," said Avery Ng of the League of Social Democrats.

"Even if the bill is vetoed, it's not a victory. We still don't have real democracy."

More than 10 pro-democracy lawmakers gave speeches at the legislature vowing to block the government's bill, to the cheers of protesters.

The rally ended in the early evening, more than an hour before the scheduled finish.

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.

Despite fragmentation in the pro-democracy camp all the key players from last year's protests are set to take part.

The latest figures from one joint university poll showed those against the reform package taking the lead for the first time with 43 percent, versus 41.7 in support.

- No quick fix -

Marchers held placards demanding "true universal suffrage" and yellow umbrellas, the symbol of the pro-democracy movement.

Protester Carol Chow, a writer in her forties, said she was not disappointed at the low turnout.

"I think it's important to keep coming. We're fighting for our democracy," she said.

Last year's mass protests saw sporadic violence, with thousands joining the demonstrations after police fired tear gas.

With the reform package expected to be blocked by pan-democrats, who have enough votes to stop it, analysts say there is little hope of a quick resolution to the political impasse.

"If the Beijing and Hong Kong governments continue to adopt hostile attitudes towards the opposition, it will be difficult for them to change the situation," said Ma Ngok, associate professor at Chinese University's department of government and public administration.

There is also pressure on the democracy movement, said Ma.

"What they can do to bring genuine democracy is a challenging question for them."

Hong Kong's chief executive Leung Chun-ying urged lawmakers to support the proposal Sunday.

"If we lose the opportunity this year, we don't know when (the next chance for reform will be)," he told reporters.

Hong Kong, which was handed back to China by Britain in 1997, has much greater freedoms than the mainland but there are fears that these are being eroded.