Tens of thousands of Chinese gathered in sweltering heat in Hong Kong on Sunday to protest against a pro-democracy campaign that has threatened to shut down the city's financial district, exposing a deepening rift over political reforms in the former British colony.

Beijing has allowed Hong Kong — returned to Chinese rule in 1997 — to go ahead with a popular vote in 2017, but who gets to run in those elections has yet to be determined. Pro-democracy groups have called on Beijing to allow open nominations rather than letting only "patriotic" pro-Beijing candidates to stand.

Hong Kong University researchers estimated that up to 88,000 people took part in the protest, about half the number they counted at a pro-democracy march on July 1.

Inspired by the Occupy Wall Street movement, the earlier pro-democracy demonstration ended with the arrests of hundreds who staged a sit-in in the financial district.

Occupy Central's organizers have sparked unease in Beijing with their plan to rally at least 10,000 people to freeze the specially administered Chinese financial hub's central business district if Beijing does not allow truly democratic elections for a leader in 2017.

The rise in tit-for-tat street protests between pro-Beijing and pro-democracy groups underscores the challenges China faces in shaping its vision for the political future of Hong Kong.

One of the Occupy leaders told local newspaper South China Morning Post that while he respects his opponents’ right to demonstrate, “I hope they can understand that Occupy Central will only be our last resort.”

Backed largely by Beijing-friendly groups, the Alliance for Peace and Democracy says it "desires peace and no violence.”

"We want to show that the march doesn't have to be violent and angry. It can be happy," said Robert Chow, a former Hong Kong radio host and spokesman for the alliance.

The alliance says it has so far collected close to 1.5 million signatures, including that of Hong Kong leader Leung Chun-ying, saying the "illegal" Occupy campaign would tarnish Hong Kong's reputation and hurt business.

It wasn't possible to independently verify the number, which exceeded the almost 800,000 votes in Occupy Central's unofficial poll on democracy in June.

The anti-Occupy campaign kicked off early on Sunday with a run through the center of the city that attracted about 1,500 in temperatures as high as 86 degrees Fahrenheit, organizers said. Protesters sang Chinese Communist Party anthems and “Do you hear the people sing” from Broadway musical “Les Miserables,” according to Hong Kong tabloid Apple Daily.

The all-day rally, which marks the end of the alliance's month-long signature campaign, was overshadowed by speculation that some business groups had pressured people to take part.

Retiree Law Kwai-wing, 77, said he had traveled across the border from Guangdong province as part of a bus tour organized by the pro-Beijing Federation of Trade Unions (FTU)

"We do not support Occupy Central because it will bring trouble and instability to the city," Law told Reuters.

Many rally participants, mostly elderly, told Reuters they had been provided with free transport by various political and business groups. In one district, about 150 people boarded buses organized by the Hong Kong Livestock Industry Association.

In a Whatsapp message seen by Reuters, people were offered HK$350 ($45) to attend the rally "for five hours." The message sender, however, declined to provide his name or background.

Alliance spokesman Chow dismissed such messages as fake and attempts to discredit the campaign.

Debate has raged over the format of the election for Hong Kong's next leader in 2017.

In March, Zhang Dejiang, the head of China's National People's Congress, told Hong Kong delegates attending the annual NPC meetings in Beijing that Western-style democracy would need to be adapted to Hong Kong.

In June, Beijing issued a white paper saying that “the high degree of autonomy enjoyed by Hong Kong is subject to the central government’s authorization.”

Al Jazeera and wire services