Tens of thousands have protested in Hong Kong against plans by pro-democracy activists to paralyse the city centre with a mass sit-in unless China grants acceptable electoral reforms.

Public discontent in the semi-autonomous Chinese city is at its highest for years, with concern at perceived interference by Beijing and growing divisions over how Hong Kong's leader should be chosen in 2017 under the planned reforms.

Pro-democracy campaigners from the Occupy Central group have pledged to mobilise protesters to block roads in the Central financial district later this year if authorities reject the public's right to nominate candidates for the chief executive post.

I am here to oppose Occupy, as simple as that. It is a bad thing for young people. Wong, protester

But the movement has been strongly criticised by Beijing and city officials as illegal, radical and potentially violent.

The Alliance for Peace and Democracy, backed largely by Beijing-friendly groups, organised Sunday's rally. It said the silent majority of the city's seven million residents do not support the Occupy movement.

"We want to let the world know that we want peace, we want democracy, but please, do not threaten us, do not try to turn this place into a place of violence," alliance co-founder Robert Chow told the AFP news agency.

Police estimated that about 110,000 people took off from Victoria Park, many of them wearing red clothes and waving Chinese.

"I am here to oppose Occupy, as simple as that. It is a bad thing for young people," a 70-year-old retired chef, who only gave his surname Wong, said.

"I don't know how to give a view on democracy, it's high-level politics. I just know if there is no peace there is no prosperity," a 40-year-old construction worker surnamed Kwok said, while holding a Chinese flag.

Many rally participants, mostly elderly, said they had been provided with free transport by various political and business groups.

'Stepped over line'

Chow said the Occupy movement had "stepped over the line".

"What they are trying to say is that if China does not bow to them, then they will occupy Central, they are going to turn the whole place into some sort of a battleground," he said.

The British colony was handed back to China in 1997 under an agreement that guarantees freedoms and civil liberties including the right to protest.

Hong Kong's leader is currently chosen by a pro-Beijing committee.

China says that all residents will be allowed to vote for the next chief executive in 2017 but that a nominating committee must choose the candidates.

Pro-democracy advocates say this means Beijing will be able to ensure a sympathetic slate of candidates.

An unofficial referendum organised by Occupy activists saw the majority of 800,000 people who voted supporting reform packages that would allow public nomination.

Organisers said a July pro-democracy march following the referendum was joined by over half a million. Statisticians from the University of Hong Kong estimated between 122,000 and 172,000 people took part.

In a counter move, an Alliance petition campaign supported by pro-Beijing groups and officials has so far collected some 1.4 million signatures, according to the group.