(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.
A massive pro-democracy demonstration had originally been planned for Saturday, but was canceled after organizers failed to secure authorization from the police.
A week of escalation
The protests followed a week of escalation, with a sudden wave of arrests on Friday of pro-democracy leaders and lawmakers -- including youth leaders Joshua Wong and Agnes Chow -- and violent attacks on a protest organizer and an off-duty policeman.
There has also been increasing speculation about Beijing's strategy toward the protesters. On Friday, Reuters reported that China had quashed a proposal by Hong Kong chief executive Carrie Lam to entirely withdraw the controversial extradition bill that sparked this summer of protest. Lam has refused to rule out invoking emergency powers.
China sent fresh troops to the city, in what it described as a routine rotation at established garrisons in the city. Beijing has stationed troops in Hong Kong since it assumed sovereignty over the city in 1997.
On Friday however, one US official told CNN that Washington had no indication that any troops had left the city, suggesting that the week's deployment may have swelled Hong Kong's garrison by the thousands. CNN has reached out to Hong Kong and Chinese authorities for comment.
Also on Friday, US president Donald Trump credited his trade talks with China for "keeping the temperature down" in Hong Kong. "I think if it weren't for the trade talks, Hong Kong would be in much bigger trouble. I think it would've been much more violent," he said. A punishing new round of tariffs will go into effect on Sunday.
Discussing the protests, Trump said, "Look we all want liberty. We all want freedom."
"I think we're going to be worrying a lot -- actually ... I think we're going to be learning a lot in the next two or three days. And I hope that it's handled in a very humane way," he added.
Online organizing platform LIHKG also said on Saturday morning that its server had received an unprecedented distributed denial of service (DDoS) attack it said was designed to drive it offline. Another key platform, the encrypted messaging app Telegram, has previously said it was targeted by DDoS attacks, which it linked to the protests.
Almost three months of protests
Hong Kong has seen protests for almost three months, with some demonstrations drawing estimated crowds of over 1 million.
Protests began in June, sparked by a controversial extradition bill that would have allowed Hong Kong to send suspected criminals to mainland China, where the legal system has a 99% conviction rate. The bill was shelved, though not withdrawn entirely. In the meantime, protesters' demands have expanded to include calls for its full withdrawal, universal suffrage, and an independent inquiry into police brutality.
Last weekend's protests were among the most violent of the summer, with some protesters armed with metal poles and petrol bombs. Police fired tear gas, water cannons and, in one case, shot a gun into the air. It was the first time that water cannons had been used in these protests.
Police have not revealed the names of all activists arrested in this week's round-up. However, they did announce that in addition to Wong and Chow, police also arrested Rick Hui, an outspoken pro-democracy politician, and Andy Chan, founder of the outlawed Hong Kong National Party.
Legislators Au Nok-hin, Cheng Chung-tai and Jeremy Tam were also arrested, said their staff and political parties on Saturday.
Among other charges, Wong and Chow, both 22, were accused with "inciting others to participating in unlawful assembly" -- which carries a maximum penalty of five years in jail if convicted. They have been released on bail.
Chow described the arrests as an attempt to tamp down on protest participation. "We can see clearly that the regime and the HK government is trying to create a White Terror to try to scare Hong Kong people to not participate in the social and democratic movement of the future," she told reporters outside the courthouse on Friday.
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