HONG KONG (AP) -- Hong Kong authorities broadened their enforcement of a new national security law on Monday, arresting media tycoon Jimmy Lai, searching the headquarters of his Next Digital group and carting away boxes of what they said was evidence.

Two days after Chinese and Hong Kong officials shrugged off sanctions imposed on them by the US, the moves showed China's determination to enforce the new law and curb dissent in the semi-autonomous city after months of massive pro-democracy demonstrations last year.

The police action marked the first time the law was used against news media, stoking fears that authorities are suppressing press freedom. Next Digital operates Apple Daily, a feisty pro-democracy tabloid that often condemns China's Communist Party government. Last year, the newspaper frequently urged readers to take part in the anti-government protests.

In Beijing, China announced sanctions on 11 Americans, including six members of Congress, in an escalating battle between the two nations over technology, security, trade and human rights. And in Chinese-claimed Taiwan, US Health and Human Services Secretary Alex Azar became the highest-ranking American official to visit  since 1979, further straining US-China relations.

Hong Kong police arrested Lai on Monday morning, an aide to the businessman said, in the highest-profile detention under the new law since it took effect in late June. Lai, 71, is an outspoken pro-democracy figure who regularly criticizes China's authoritarian rule and Hong Kong's government.

Mark Simon, a Next Digital executive and Lai's aide, said Lai was charged with collusion with foreign powers. He said police searched the homes of Lai and his son and detained several other members of the media company.

Hong Kong police said they arrested at least nine people between the ages of 23 and 72 on suspicion of violating the new security law, with offenses including collusion with a foreign country and conspiracy to defraud. They did not release the names of those arrested or provide further details of the charges.

Following Lai's arrest, about 200 police raided Next Digital's headquarters, cordoning off the area, searching desks and at times getting into heated exchanges with staff. What police were looking for in the building wasn't clear, although they later said they took away 25 boxes of evidence for processing.

Lai, who was arrested at his mansion in Kowloon in the morning, was also brought to the headquarters of Next Digital, where he remained for about two and a half hours before police took him away in a car.

"We are completely shocked by what's happening now, with the arrest and followed by the ongoing raid inside the headquarters of Next Digital," said Chris Yeung, chairman of the Hong Kong Journalists Association.

"With the passage of the national security law and the really tough powers given to the police in their operations, we have seen now what we call `white terror' become a reality, which will affect media organizations and journalists' reporting."

Police unblocked Next Digital's headquarters at mid-afternoon, with senior superintendent of police Steve Li saying that staff were free to resume their work.

Bruce Lui, a senior lecturer in Hong Kong Baptist University's journalism department, said authorities are using the national security law to make an example of media outlets like Apple Daily and this may harm press freedom in Hong Kong.

"They're used as an example to terrify others ... of what can happen if you don't obey or if you go too far," Lui said. "I think other media may make a judgment to censor themselves."

The share price of Next Digital soared over 200% in the afternoon, following posts on a popular online forum encouraging investors to support the company by buying its stock.

The reason for the charge against Lai wasn't clear.

In May, shortly after Beijing announced its intention to pass the national security law for Hong Kong, Lai condemned the legislation in a series of tweets. The state-owned newspaper Global Times called the tweets "evidence of subversion."

Lai also wrote an op-ed in the New York Times in May stating that China was repressing Hong Kong with the legislation.

"I have always thought I might one day be sent to jail for my publications or for my calls for democracy in Hong Kong," Lai wrote. "But for a few tweets, and because they are said to threaten the national security of mighty China? That's a new one, even for me."

Lai was earlier arrested in February and April for allegedly participating in unauthorized protests last year. He also faces charges of joining an unauthorized vigil on June 4 marking the anniversary of Beijing's crackdown on pro-democracy protesters in Tiananmen Square in 1989.

Last year, Lai met US Vice President Mike Pence and Secretary of State Mike Pompeo at the White House to discuss a controversial bill _ since withdrawn _ that would have allowed criminal suspects in Hong Kong to be sent to mainland China for trial.

But Hong Kong officials have said the security law, which took effect June 30, would not be applied retroactively. The law is widely seen as a means to curb dissent after anti-government protests rocked the semi-autonomous city for months last year.

The legislation outlaws secessionist, subversive and terrorist acts, as well as collusion with foreign forces in the city's internal affairs. The maximum punishment for serious offenders is life imprisonment.

Taiwan's Mainland Affairs Council condemned the arrests in a statement, saying they were a tool for the Chinese Communist Party's "political cleansing and hegemonic expansion." It said the law is being abused to suppress freedom of speech, press freedom and the civil rights of Hong Kong people.

Last month, Chinese state broadcaster CCTV said pro-democracy activist Nathan Law and five others were wanted under the law, although all six had fled overseas. Law relocated to Britain in July to continue international advocacy work for Hong Kong. (AP)