Hong Kong's leader declared a planned occupation of the city's financial district by pro-democracy campaigners illegal Tuesday, and warned senior officials not to support the movement.

Public discontent in Hong Kong is at its highest for years, with heightened concern over perceived interference from China, notably Beijing's insistence that it vet candidates before the vote for the city's next leader in 2017.

Occupy Central campaigners have pledged to mobilise protesters to take over some of the busiest thoroughfares of the former British colony to push for electoral reform.

But the city's chief executive Leung Chun-ying accused the group of "using the illegal means of paralysing Central (business district) to coerce the central authorities to accept a political reform proposal they have demanded".

Speaking at a press conference he said: "I think any senior official or any law-abiding citizen should be opposed to such large-scale criminal actions."

"There is no grey area between abiding by the law and breaking the law," he added.

An informal democracy poll organised by Occupy garnered almost 800,000 votes in June after being criticised by state-run media as an "illegal farce".

Voters chose from three options for electoral reform in the civil referendum, all of which included public nomination of candidates for the next leadership elections.

China says voters can choose the next city chief executive but candidates must be picked by a nominating committee -- raising fears among democracy advocates that only pro-Beijing figures will be allowed to stand.

Carrie Lam, Hong Kong's Chief Secretary for Administration and its second highest official, said Tuesday after meeting with Occupy Central organisers that Beijing would not be moved by the street campaign.

"It would not be possible to try to coerce the Central Authorities into a particular position on this important subject of selection of the Chief Executive by threatening to do some sort of action like Occupy Central," Lam told reporters.

Occupy would "cause a lot of disruptions," she said.

Lam was the most senior official to meet with the organisers of the campaign, which has been heavily criticised by Beijing.

Benny Tai, an Occupy leader, said after the meeting with Lam that the occupation would go ahead if the government failed to propose an acceptable reform package.

"It is a political reality that the Occupy Central campaign will happen if there is no democracy that complies with international standards," he told reporters.

"It is a bit disappointing that the government did not offer concrete plans to resolve the constitutional crisis," he said.

Earlier this month, Leung called for limited electoral change, saying in a report to China that voters want a "patriotic" chief executive.

The city was handed back to China in 1997 under an agreement which guaranteed rights such as freedom of speech and an independent judiciary.