The sentence of three Hong Kong pro-independence activists on Monday is a blow to pro-independence forces that intend to achieve their goals through violence, observers said.

Edward Leung, 27, the former convenor of political group Hong Kong Indigenous, was sentenced by Hong Kong's High Court to six years in jail after pleading guilty to assaulting a police officer during the Mong Kok riot in February 2016.

Two other defendants in the case, Lo Kin-man and Wong Ka-kui, were sentenced to seven and three-and-a-half years, respectively, for rioting, according to Reuters. It wasn't clear if Leung would appeal.

 "It is a blow to pro-independence Hong Kong people who once hailed him as a hero and supported Leung. Although it doesn't directly target the issue of independence, the sentence is a warning to people who try to violently pursue their political goals," Yin Hongbiao, a Peking University professor, told the Global Times on Monday.

In contrast to the illegal "Occupy Central" movement in 2014, in which suspects were only given a light punishment, the Mong Kok riot saw people being sentenced much heavier. It means that Hong Kong has "zero tolerance against violence," added Tian Feilong, an assistant professor at Beijing-based Beihang University.

One week before Leung's sentence, two other pro-independence activists and former lawmakers were sentenced to one month in jail for an illegally assembling in the legislature while still in public office, Reuters reported.

Over 90 police officers were injured in a clash with rioters following a clearing operation of illegal hawkers in Mong Kok, a busy commercial district in Hong Kong.