Hong Kong (CNN) Police and protesters faced off in Hong Kong for the 13th consecutive weekend of pro-democracy protests, amid plumes of tear gas and scattered fires. Over a long night of violent clashes, police fired two warning shots skyward, and rolled out a water cannon for the second time this summer. Protesters threw petrol bombs and other objects.

At least 51 people were arrested in the streets and in the subway, police said on Sunday. Video footage from Prince Edward subway station in Kowloon showed officers chasing and hitting individuals with batons as they made the arrests. Arrestees have been accused of "participating in unauthorized assembly," "criminal damage," and other charges.

The police response was quicker and fiercer than in past weeks, perhaps in a sign of waning patience, and clashes continued until the very early hours of Sunday morning local time, where angry crowds gathered outside of Mong Kok police station. Hundreds could be heard shouting and jeering, and across the street, cars honked to show their support for the protesters.

Earlier in the day, thousands had taken to the streets for a series of wildcat protests. Police had barricaded major streets around the China Liaison Office, which represents the mainland government, but it was at the main Hong Kong government offices that protests took a more violent turn.

Confrontations between protesters and police moved through the city -- at one point punctuated by a large bonfire of piled barricades and debris, which police quickly extinguished -- as day turned to night.

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