A police officer was struck by an arrow fired by a Hong Kong protester on Sunday, as fierce clashes raged around a campus which has turned into a base for a pro-democracy movement whose unrelenting protests have plunged the city into turmoil. Protests have tremored through the global financial hub since June with many in the city of 7.5 million people venting fury at eroding freedoms under Chinese rule. Chinese President Xi Jinping this week issued his most strident comments on the crisis, saying it threatened the “one country, two systems” model under which Hong Kong has been ruled since the 1997 handover from Britain.