HONG KONG — Police fired rounds of tear gas, used a water cannon to disperse protesters and arrested more than 100 people opposed to Beijing's plan to impose a sweeping national security law in a return to the demonstrations that defined this city last year.

Despite social distancing measures that prohibit gatherings of more than eight and laws on illegal assembly, tens of thousands of people thronged through Hong Kong’s Causeway Bay shopping district on Sunday afternoon. Calls to assemble were made online without a formal organizer or permit.

Refrains of last year’s protests — “fight for freedom,” “stand with Hong Kong” — echoed on the streets among people of all ages, along with some newer ones: “Hong Kong independence, the only way out.” Some carried posters declaring that “heaven will destroy the Chinese Communist Party.”

“If we don’t come out today to fight back, this may be the last time,” said Chris, a 19-year-old protester who spoke on the condition that only his first name be used because he had been arrested once before for participating in an illegal protest. “Maybe tomorrow, Hong Kong will be China, and we can’t even say a single word of criticism on the Internet without being arrested.”

Some activists gathered near the starting point of the planned march under the auspices of a “health talk,” claiming they were exempt from restrictions on public gatherings. They were arrested nonetheless for unauthorized assembly.

The protest soon descended into familiar scenes: bottles thrown at police, rounds of tear gas fired in response, cat-and-mouse tactics between protesters and officers, and eventually arrests. By Sunday night, police said they had arrested at least 180, mostly on charges of unlawful assembly.

The new national security law will criminalize “foreign interference,” secessionist activities and subversion of state power. Beijing plans to impose it by decree, bypassing the legislative processes set up in Hong Kong by the 1997 handover from Britain.

The move undermines Hong Kong’s constitution, the Basic Law, and essentially discards the “one country, two systems” approach meant to preserve the city’s autonomy until at least 2047.

U.S. Secretary of State Mike Pompeo has called the law “a death knell” for Hong Kong’s autonomy. National security adviser Robert C. O’Brien said Sunday that a Chinese effort to assert dominance over Hong Kong would draw U.S. financial sanctions against both.

Speaking on NBC’s “Meet the Press,” O’Brien predicted that global financial firms and well-educated residents would flee Hong Kong.

“It’s hard to see how Hong Kong can remain the Asian financial center it’s become if China takes over,” he said.

In Beijing, Chinese Foreign Minister Wang Yi said Sunday that legislation was “urgent and imperative” after the protests that erupted last year.

“These protests had posed a grave threat to Hong Kong’s prosperity and stability and to the practice of ‘one country, two systems,’ ” Wang said at a news conference on the sidelines of the annual National People’s Congress.

He said establishing a new legal system and enforcement mechanisms was “a pressing priority” and China “must get it done without the slightest delay.”

But he characterized the new law as narrowly defined.

“This has no impact on Hong Kong’s high degree of autonomy, the rights and freedoms of Hong Kong residents, or the legitimate rights and interests of foreign investors in Hong Kong,” he said.

Just 20 minutes after the march Sunday was due to start, protesters had spilled onto roads, snarling traffic in scenes reminiscent of the months of sometimes violent unrest last year.

Police moved quickly to disperse them, firing several rounds of tear gas into the crowd, leaving the elderly coughing and choking and parents rushing children into malls for safety as the shutters closed on them.

At one point, police fired a water cannon mounted on an armored truck at protesters and then into the air.

Even after most of the activists scattered, police remained on the streets of downtown. Some people shouted “communist thugs” at them.

In some cases, protesters attempted to build barricades in the streets to hinder police movement — a hallmark of last year’s protest — but they failed to hold them back for long. A heavy riot police presence blocked dozens of roads and fortified subway stations in a show of force across the heart of the city.

Police said that the demonstrators were “causing serious obstruction to the road traffic” and that they used “minimum necessary force” to disperse them.

“Police do not condone the unlawful and violent acts of rioters and warn against all disruptions that endanger public safety and breach the public peace,” police said in a statement.

A spokesman for the Hong Kong government condemned “violent acts” and labeled protesters advocates of independence. The events show “rioters remain rampant, reinforcing the need and urgency of the legislation on national security,” the spokesman said.

The clashes appeared to mark the beginning of what will probably be a long and restive summer in Hong Kong, as thousands pledge to never give up the fight against Beijing’s control, despite their diminishing power.

Protesters say they want to continue living with the freedoms they have always enjoyed rather than being subsumed into mainland China.

The turmoil erupted last year with the introduction of a bill that would have allowed criminal extraditions to mainland China. The protests grew into a political movement pushing back against Beijing’s encroachment on the city’s political freedoms.

Under the 1997 agreement, Hong Kong administers its own affairs ­— save defense and foreign relations — and has become a thriving center for finance and media.

“I’m worried about my future, but it is pointless to think of that now,” said Fong, a protester who spoke on the condition that only her last name be used for fear of arrest. “I know we have no power against Beijing, but we still have to stand up and say to the world that this is bad for Hong Kong.”

Anna Fifield contributed to this report.