HONG KONG—The night before Christmas was marked with tear gas and rubber bullets as police tried to disperse protesters gathered near the city’s harbor front, signaling a renewed escalation in the conflict after a few weeks of relative calm.

Hundreds gathered in the tourist-heavy neighborhood of Tsim Tsa Tsui to chant “fight for Hong Kong” and “five demands.” Around 9 p.m., riot police fired several rounds of tear gas near the Peninsula hotel, a luxury British colonial-era establishment that has been hit hard by falling numbers...