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HONG KONG — Around 400 protesters were arrested after a police-approved march attended by tens of thousands of people on New Year's Day escalated into violence, authorities said.

The arrests, which police said were for unlawful assembly and possession of offensive weapons, came after officers fired several rounds of tear gas as anti-government protests in the semi-autonomous city continued into the new year.

The Hong Kong broadcaster RTHK showed pepper spray and tear gas being deployed against demonstrators in the busy commercial district of Wan Chai on Wednesday afternoon as protesters marched from Victoria Park into the central business district.

Anti-government protesters in Hong Kong continued their demands in 2020. Anthony Kwan / Getty Images

Hong Kong police said in a statement that some protesters hurled Molotov cocktails and vandalized banks and shops along the route of a public procession, which forced them to end the march prematurely and tell the crowds to disperse.

In a further escalation Wednesday evening, some protesters dug in, barricading roads, starting fires and pointing lasers at officers, prompting police to deploy a water cannon and more tear gas.

Earlier in the day, residents young and old marched in a largely peaceful rally, carrying signs that read "Five demands, hold on till the end" and "Never give up." Many wore masks to conceal their identities.

Sandy Lo attends an anti-government rally with her young son on Wednesday, Jan. 1, 2020. Their banner reads, 'Liberate Hong Kong, revolution of our time.' Mark Roberts / for NBC News

"Hong Kong needs its freedom. It's one country, two systems — not one country, one system," said Sandy Lo, a homemaker who came to the march with her young son, referring to the unique model that governs the former British territory and guarantees freedoms not granted in mainland China.

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"We hope that [Hong Kong leader] Carrie Lam listens to our voice," she said.

Several protesters who were carrying U.S. flags told NBC News that they were grateful for President Donald Trump's backing of legislation to support human rights in Hong Kong — a move that enraged Beijing.

"We deeply appreciate it and are deeply grateful for his support," said Claudia Chin, 26, a sales manager. "We are here to thank him."

Wearing big sunglasses and a mask, she added that she hoped 2020 would be the year the protesters' five demands are met, among them universal suffrage.

An anti-government protester paints graffiti during a demonstration on New Year's Day to call for better governance and democratic reforms in Hong Kong. NAVESH CHITRAKAR / Reuters

For months, demonstrators have been pushing for concessions from the government, including full democracy, a declaration of amnesty for protesters who have been arrested and an independent investigation into police actions.

"This is the year that we really hope these demands will come true," Chin said.

Hong Kong toned down its New Year's Eve celebrations amid the protests, which have dragged on since June.

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The months of civil unrest were sparked by a controversial bill that would have allowed residents to be extradited to China. Although the bill has been shelved, the demonstrations have transformed into a wider movement against the erosion of civil liberties that were promised after the city was handed over to China in 1997.

Beijing has denied that it's interfering in Hong Kong's affairs, blaming the West for fomenting the unrest.

A recent poll conducted for Reuters by the Hong Kong Public Opinion Research Institute indicated that the protest movement is supported by 59 percent of residents, with more than a third of respondents saying they had attended an anti-government demonstration.

However, protests have grown increasingly violent in recent months, affecting Hong Kong's reputation as a financial capital and a tourism hot spot.

In her New Year's address, Lam said the months of protests had brought "sadness, anxiety, disappointment and even rage," and she vowed to tackle underlying social and economic problems in the coming year.

Lam said she would "listen humbly" to help end the protests.

On Wednesday, lawmakers and civic leaders from 18 countries urged Lam to set up an independent investigation into police use of force in the protests, warning that they would otherwise call for an inquiry at the international level, the South China Morning Post reported.

Meanwhile, Chinese President Xi Jinping said in comments broadcast Tuesday that he "sincerely wishes Hong Kong and our Hong Kong compatriots well," adding that the situation has been a concern.

"Without a harmonious and stable environment, how can people live in peace and enjoy their work?" Xi asked in a New Year's Eve speech carried on state television. "Hong Kong's prosperity and stability is the wish of Hong Kong's compatriots, and it's also the expectation of the people of our motherland," he added.

Mark Roberts and Veta Chan reported from Hong Kong. Yuliya Talmazan reported from London.