HONG KONG — Protesters armed with bows and arrows on Thursday reinforced the siege-like fortifications they had built on Hong Kong university campuses in anticipation of clashes with the police, and a 70-year-old man died after being struck in the head, possibly by a thrown brick.

President Xi Jinping also gave his toughest public comments yet about the protests that have roiled Hong Kong since June, saying China supported the police “in sternly enforcing the law.”

The protests started over an extradition bill that has since been withdrawn, and have morphed into broader demands for democracy and police accountability.