Political activist Ken Tsang vowed to appeal the sentence over the liquid splashing incident, which happened the same night he was allegedly beaten by another group of police in a brutal attack captured by television cameras and beamed around the world.
Tsang's is the first conviction for a high-profile figure involved in the rallies, dubbed the "Umbrella Revolution", which blocked major highways in the city for over two months, with protesters calling for a free vote on the city's leader.
"We are going to stand until the last minute. We are going to fight until the last minute. We will never give up," 40-year-old Tsang told reporters, adding he would appeal the sentence.
Tsang was given concurrent sentences -- five weeks for one count of assaulting police and three weeks each for two counts of resisting officers during his arrest near the government's headquarters. He has been bailed pending appeal.
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During sentencing, magistrate Peter Law said Tsang showed "extreme insult and provocation" towards police officers.
"Up to now he has shown no sign of remorse... I believe a jail term is the only option and it is indeed appropriate," he told the courthouse, which was packed with Tsang's supporters.
Around 30 supporters, some carrying yellow umbrellas -- a symbol of the democracy movement -- protested outside the courthouse, shouting "shameful police" and "I want true universal suffrage".
Tsang has said police brought assault charges against him to distract from the case against them.
The police who allegedly beat him, who are not the same officers Tsang is convicted of assaulting, are to stand trial separately.