Hong Kong student leader Joshua Wong and another prominent young pro-democracy activist were charged Tuesday over an anti-China protest last year, in what they said was a witch hunt against political campaigners in the city.

Wong, 18, the teenage face of the city's pro-democracy movement, and Nathan Law, 22, leader of the major university student union, accused the authorities of rounding up activists 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.

The charges against Wong and Law date back to a small peaceful protest in June last year, before large-scale pro-democracy rallies brought parts of the city to a standstill.

They were among dozens who 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.

Wong emerged from a Hong Kong police station after more than three hours and said he had been charged with two counts of obstructing police officers.

"It's political persecution... It's mystifying that I am to be charged for being part of a legal protest," Wong said.

Lawyer Michael Vidler, who represents Wong, questioned the delay in arresting and charging the student when his whereabouts where known to the authorities.

"It all gives rise to the suspicion that this is persecution rather than proper policing," he told AFP.

"It may be in due course we will make an application that this is abuse of process."

Law was charged on one count of obstructing police officers and said it set a dangerous precedent.

"If we are charged because we burned the white paper, it means protesters can face more clampdowns in future when they oppose Hong Kong or central government policies," he said.

Police made no immediate comment.

Wong and Law will appear in court Friday with two other activists -- Raphael Wong and Albert Chan -- who have already been charged for obstructing police officers at the June protest.

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.

It 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.