Hong Kong student leader Joshua Wong and three other pro-democracy activists entered "not guilty" pleas in court on Friday over charges related to an anti-China protest last year.

They have accused the city authorities of a witch hunt against political campaigners after a contentious Beijing-backed reform package was vetoed last month.

The rejection of the government's bill was an unprecedented rebuke to Beijing and left the city politically polarised.

Wong, 18, the teenage face of the pro-democracy movement, is charged with obstructing police at a small peaceful protest in June 2014 -- before large-scale democracy rallies gripped the city.

Nathan Law, 22, leader of Hong Kong's major university student union, and activists Raphael Wong and Albert Chan -- a legislator for the People Power party -- appeared on the same charges.

They were among dozens who had gathered outside Beijing's representative office in Hong Kong to oppose a "white paper" from China that asserted its control over the semi-autonomous city and a reproduction of the document was burned.

"It is not a crime to burn the white paper -- shame on political suppression," said Raphael Wong, a member of the League of Social Democrats party, as he entered his plea, ripping up a copy of the white paper, before being warned by judge Bina Chainrai not to use the courtroom to express his political views.

Joshua Wong spoke only to confirm his plea.

"Joshua Wong is not guilty," he said.

Around 20 protesters carrying yellow umbrellas gathered outside the court and burned a copy of the white paper, shouting: "Return my freedom of expression, step down Leung Chun-ying," referring to the city's unpopular leader.

"This is something that happened over a year ago and there were no arrests," said Chan after the hearing.

"This is 100 percent political persecution."

Chainrai questioned why the four had been arrested and brought to court after more than a year.

"Why has it taken so long?" she asked, saying that a "stay" of proceedings could be sought by the defence.

The case was adjourned to August 28 for a pre-trial review.

It comes weeks after lawmakers delivered a slap in the face to Beijing by rejecting the reform package, which would have allowed the public to vote for the city's leader for the first time in 2017.

The bill was vetoed last month by opposition legislators who derided it as "fake democracy" because it adhered to a Beijing ruling that all candidates must be vetted by a loyalist committee.

That ruling sparked more than two months of street rallies towards the end of last year during which hundreds of protesters, including prominent politicians, were arrested, though few have been charged.

Hong Kong was handed back to China by Britain in 1997 and is largely self-ruling, but there are fears its freedoms are being eroded.

Wong was attacked in the street last month raising concerns that the city's deep political divisions could turn violent.

Leading media figures in the city have also been physically assaulted.