A key figure in Hong Kong's "Umbrella Revolution" has been blasted as a US-backed agent intent on sowing dissent and discord in a new propaganda video from the Chinese government.

The two-minute film targets those mainland authorities believe threaten Beijing with student leader Joshua Wong featuring twice and images of US president Barack Obama, the Dalai Lama and Taiwan's President Tsai Ing-wen also shown.

It comes as tensions mount ahead of Hong Kong's legislative elections where at least four candidates who want the semi-autonomous city to break from the mainland have been banned from standing, prompting anger the vote is being vetted.

Wong, who has been convicted of participating in 'unlawful assembly' -- the protest that led to mass pro-democracy rallies in 2014 -- is awaiting sentencing later this month.

Viewed as the public face of the "Umbrella Revolution", Wong and other high-profile pro-democracy activists have launched new political party Demosisto, promoting self-determination for Hong Kong.

Many frustrated young campaigners in the city are seeking more distance from Beijing after pro-democracy protests in 2014 failed to win political reform from China.

The propaganda video features public figures unpopular with Beijing as it warns against forces who want to destabilise China.

It was first posted Monday on the verified weibo account of the Supreme People's Procuratorate of the People's Republic of China -- the national prosecuting authority -- and on the Communist Youth League of China's account Tuesday.

"(Figures behind) Tibet, Xinjiang, Hong Kong and Taiwan independence...and the agents of western power are doing whatever they can to destroy China's stability and harmony," video subtitles read.

"Western powers led by the US, use (the ideas of) democracy freedom and the rule of law to create social conflict in targeted countries."

With a dramatic orchestral soundtrack the video starts with footage of traumatised children in war zones, questioning what will happen to Chinese youngsters if "external and internal" threats prevail.

It then flashes images including anti-government rallies in Taiwan and the island's new Beijing-sceptic leader Tsai, as well as pictures of Hong Kong protesters and Wong.

Wong, 19, mocked the video.

"I just consider it a joke after watching it," Wong said in a post on his Facebook page.

"The Communist Party best not think that by writing 'Hong Kong independence' in the subtitles, it can...suppress dissent," he said.

Wong is due to be sentenced on August 15.

His conviction was slammed by rights activists as undermining freedom of expression.

Hong Kong was returned from Britain to China in 1997 under an arrangement that guarantees civil liberties unseen on the mainland.

But concerns have grown that such freedoms are now fading as Beijing increases its influence across a range of areas, from politics to the media.