Hong Kong student leader Joshua Wong slammed police Thursday for what he called a "political prosecution" after he was charged over protests which led to last year's mass pro-democracy street rallies.

Wong, 18, who became the face of the democracy movement, is already due in court Friday for separate charges of obstructing police at an earlier demonstration.

The teenager and other activists have accused the authorities of a witch hunt against those at the forefront of the Umbrella Movement that brought parts of the city to a standstill for more than two months.

Protesters called for fully free elections of the city's next leader, in the face of a contentious Beijing-backed political reform package in which candidates would have been vetted by a loyalist committee.

That bill was vetoed in June in an unprecedented rebuke to Beijing, leaving the city politically polarised.

Wong was charged Thursday with "unlawful assembly, and inciting others to take part in an unlawful assembly".

The charges, which could result in a sentence of up to five years, relate to a student protest on September 26 last year during which some climbed into a square which is part of the city's government complex.

Wong and other activists were arrested, sparking wider demonstrations which exploded two days later when police fired tear gas to disperse the crowds.

Thousands more took to the streets in the wake of what they saw as heavy-handed policing.

- No regrets -

"Today... is political prosecution," said Wong before he entered a police station Thursday in the central neighbourhood of Wan Chai.

"Being involved in the civic square action is the best mission I have made in the four years I've been involved in social student movements," he said.

"I will not regret it... even if I have to pay the price."

When they emerged less than two hours later, Wong's lawyer Michael Vidler confirmed the teen had been charged.

"I think the whole rationale for proceeding at this stage, a year after the event, is totally flawed," Vidler said.

"In my view it's a clear abuse of process."

Prominent student protester Alex Chow was also charged Thursday with taking part in an unlawful assembly.

Student leader Nathan Law was charged with inciting others to take part in an unlawful assembly.

Around 40 supporters outside the police station held up yellow umbrellas -- the symbol of the democracy movement -- and shouted "CY step down!" referring to the city's unpopular leader Leung Chung-ying.

"The police were waiting for the right opportunity so they wanted everything to die down," Surya Deva, associate professor at the School of Law at the City University of Hong Kong, told AFP.

"It was not a complicated criminal investigation... they should not be taking almost one year to prosecute someone," Deva said.

The trio will have their charges heard at a magistrates court on September 2.

Wong and Law, 22, were brought to court last month on separate charges of obstructing police at an earlier protest in June 2014.

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.

Both pleaded "not guilty" last month to those charges and will appear in court Friday for the next hearing in the case.