Tens of thousands of Hong Kong residents began marching through the streets of the former British colony to push for greater democracy in a rally fuelled by anger over Beijing's recent warning that it holds the ultimate authority over the southern Chinese financial center.
Organisers said they expect the crowd to swell to least 180,000 for Tuesday's march to press for reforms allowing residents to elect their leader. Other reports put the expected rally turnout at half a million.
The march comes after an informal poll on democratic reform drew an unexpectedly high turnout of nearly 800,000 votes, but was branded "illegal and invalid" by Beijing.
Al Jazeera's Adrian Brown, reporting from Hong Kong, said a large number of protesters have joined in the march, which started at 3pm (7am GMT).
"They are expecting half a million people, and I think they are going to get that figure," our correspondent said.
July 1 is traditionally a day of protest in Hong Kong and also marks the anniversary of the handover from Britain to China in 1997, under a "one country, two systems" agreement.
That agreement allows residents civil 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 are people protesting in Hong Kong on multiple issues right now, but mostly it's over China's refusal to allow the people of Hong Kong to have a free vote in the election that would decide, who will be the next leader of this city," Al Jazeera's correspondent said.
There has also 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.
Pro-democracy group Occupy Central, which organised the successful referendum, has said that it will stage a mass sit-in in the city's business district later this year unless authorities come up with acceptable electoral reforms.
On Sunday, a 10-day unofficial referendum was held giving three options for voters in the elections for the city's leader, all of which included the public having some influence on the selection of candidates.
Beijing condemned the vote on Monday and accused its organisers of breaching the rule of law.
Andew Shum, spokesman of the Civil Rights Front, told Al Jazeera that pro-democracy leaders "feel that is a critical point" to push for electoral reforms, saying that the universal suffrage promised by China does not meet international standards.
Ahead of the protest, a small group of people burned a copy of the white paper and a picture of the city's leader, Leung Chun Ying, after they were stopped by police near a flag-raising ceremony to mark the handover on Tuesday morning.
China has promised to let all Hong Kong residents vote for their next leader in 2017. But China said candidates must be approved by a nomination committee, which democracy advocates fear will mean only pro-Beijing figures are allowed to stand.
A study released on Monday by the Chinese University's Hong Kong Institute of Asia-Pacific Studies suggested that mistrust of Beijing is growing.
Nearly 44 percent of about 800 Hong Kong residents interviewed for the monthly survey said they did not trust the central government, up five percentage points from May.
Source: Al Jazeera and agencies