Hong Kong student leader Joshua Wong was acquitted Tuesday over an anti-China protest in the first of a series of cases against him to reach a verdict.

Wong, 19, was the teenage face of the pro-democracy Umbrella Movement, which brought parts of Hong Kong to a standstill for more than two months in late 2014 with street rallies calling on Beijing to allow fully free leadership elections.

Tensions remain high in the semi-autonomous Chinese city, with fears growing that Beijing is tightening its grip.

Around 20 supporters gathered outside court chanting and carrying yellow umbrellas, the symbol of the democracy movement.

"The result of this trial proves that it was a political prosecution," Wong said after the verdict.

"I think it proves that the core values of Hong Kong are eroding."

The teenager was in court over a small protest in June 2014 -- before the mass rallies -- which saw dozens gather outside Beijing's representative office in the city.

They were opposing a "white paper" from China that asserted its control over Hong Kong, and burned a reproduction of the document.

Wong, student leader Nathan Law, and activists Raphael Wong and Albert Chan were all charged with obstructing police, which carries a possible two-year jail term.

They pleaded not guilty and all were acquitted.

Magistrate Lee Siu-ho said Wong could not be held accountable for the fact that police were unable to control the crowd.

One officer said Wong had obstructed him from carrying out his duty by knocking an empty water bottle from his hand -- thereby preventing him from recycling it.

In his judgement Lee dismissed that argument, saying that was a mere "inconvenience".

Student leader Law criticised the justice department for allowing the case to go ahead on weak evidence.

"The most important goal was to harass us," he told reporters.

Wong recently stood trial over another demonstration that saw students climb into the Hong Kong government complex on September 26, 2014, triggering wider rallies that exploded two days later when police fired tear gas to disperse crowds.

That verdict will be given later this month.

Wong said Tuesday he was more worried about the second case, which carries a heavier sentence.

However, he added: "I still think Hong Kong... has rule of law and judicial independence."

The university student also faces a charge relating to a protest in Mong Kok, where the most violent clashes happened during the pro-democracy rallies.

Hong Kong is semi-autonomous after being returned to China by Britain in 1997, with much greater freedoms than seen on the mainland.

But there are fears those freedoms are being eroded by increasing interference from Beijing.