Hong Kong police arrested hundreds of people amid defiant protests on the anniversary of the city's handover to Chinese rule, and on the first day of a draconian new security law that was already having an impact on freedom of speech.

"Police arrested over 300 persons, including 10 people for suspected violation of the National Security Law," the city's police force said in a statement.

"The remaining arrestees were arrested for suspected unlawful assembly, disorder in public places, furious driving and possession of offensive weapon," it said.

A senior Chinese official said anyone arrested by the mainland�s new national security office in Hong Kong on charges of violating the new national security law for the city would be tried in the mainland, although it was unclear whether Wednesday's arrests were made by that office.

Zhang Xiaoming, executive deputy director of the Hong Kong and Macao Affairs Office, said China�s national security office in the city abides by Chinese law and that Hong Kong�s legal system cannot be expected to implement those laws.

The arrests came after crowds gathered in the shopping and entertainment districts of Wanchai and Causeway Bay in defiance of a police ban on the traditional annual protest march, and of the new ban on any expression of pro-independence sentiment.

Chanting: "Five demands, not one less! Fight for our freedom!" and "Hong Kong independence, the only solution!" the crowds faced down hundreds of police in full riot gear who were drafted into the area during the afternoon.

Police fired at individuals, one of whom was a journalist sent flying onto the pavement in footage captured by several social media accounts, with a water cannon truck that patrolled the streets of Wanchai and Causeway Bay as police raised flags declaring the gathering "illegal."

'Rule of law is dead'

A protester surnamed Loh said she had attended the protest to display a placard which read: "Loving Hong Kong is not a crime."

"The rule of law is dead, starting from today," Loh told RFA. "Now we daren't say anything."

"I grew up here, and it is already not the same place today that it was yesterday," she said. "I don't want the Hong Kong I knew to die, and there is no crime in my loving it."

A protester surnamed Leung said the national security law, which contains sweeping and vaguely worded bans on speech as well as action, including speech critical of the authorities or promoting independence, showed that the ruling Chinese Communist Party had abandoned all pretense over Hong Kong's promised freedoms and was taking over.

"They're not even bothering with one country, two systems any more; they are showing their true colors," Leung said. "They are imposing mainland Chinese law enforcement on Hong Kong. It's instant mainlandification."

"What does mainlandification mean? It means the loss of reasonable government, because it's the mainland we're dealing with now," he said. "The Chinese Communist Party isn't a rational entity; it's an organization of utmost evil, and it wants to make Hong Kong in its own image."

Civic Party politician Kwok Ka-ki said the law would likely also have a huge impact on the city's economy.

"Taiwanese people are already saying they will avoid Hong Kong at all costs," Kwok said. "Because someone from Taiwan would totally be targeted if they were to utter a single word against the Chinese or Hong Kong authorities, or if they were to refer to Taiwan as an independent entity."

Pro-democracy lawmaker Claudia Mo said journalists could also soon find themselves in "dire trouble" under the new law.

"Anyone giving or disseminating any [sensitive] material or information to a journalist, and this journalist publishes information obtained in such a manner, could be in dire trouble. Both of them," said Mo, who is a former journalist herself.

"This is not the rule of law. These is not even rule by law. This is rule by decree. Free press could just be announced dead in Hong Kong," she said.

U.K. offers route to immigration

In Washington, U.S. Secretary of State Mike Pompeo noted that the feared arrests are "already happening" under the new law, which he called a "violation of commitments that it made to the Hong Kong people and to the United Kingdom, in a UN-registered treaty � and in contravention of Hong Kongers� human rights and fundamental freedoms."

"Security forces are already rounding up Hong Kongers for daring to speak and think freely. The rule of law has been eviscerated. And as always, the Chinese Communist Party fears its own people more than anything else," he said at a news briefing Wednesday.

"The United States is deeply concerned about the law�s sweeping provisions and the safety of everyone living in the territory, including Americans," added Pompeo, who has recently unveiled visa restrictions and military trade restrictions in response to the Chinese policies in Hong Kong.

The U.K. said it would offer all those in Hong Kong with British National Overseas (BNO) status a "bespoke" immigration route, foreign minister Dominic Raab said in a statement after the security law took effect.

"The prime minister and the government are crystal clear: the United Kingdom will keep its word, we will live up to our responsibilities to the people of Hong Kong," Raab told parliament.

"I can now confirm we will proceed to honor our commitment to change the arrangements for those holding BNO status," he said, adding that those with such status would be granted five years of limited leave in Britain to work or study.

After that, they could apply for settled status and after a further 12 months with settled status, they would be able to apply for citizenship. There will be no cap on the numbers who may apply.