Hong Kong's Apple Daily tabloid responded with defiance on Tuesday to the arrest of owner Jimmy Lai under a new national security law imposed by Beijing, promising to "fight on" in a front-page headline over an image of Lai in handcuffs.

Readers queued from the early hours to get copies of the pro-democracy tabloid a day after police raided its offices and took Lai into detention, the highest-profile arrest so far under the national security law.

"Apple Daily must fight on", the front page headline read, amid fears the national security law amounts to an assault on freedom of speech in the semi-autonomous territory.

"Yesterday will not be the darkest day for Apple Daily as the subsequent nuisances, suppression and arrests will continue to induce fear in us," it wrote in an editorial.

"Nevertheless, the prayers and encouragement of many readers and writers make us believe that as long as there are readers, there will be writers, and that Apple Daily shall certainly fight on."

More than 500,000 copies were printed, up from the usual 100,000, the paper said on its website.

Dozens of people queued for the paper in the working-class neighbourhood of Mong Kok as early as 2:00 a.m. (1800 GMT). Some vendors said they sold out during the morning rush-hour.

"What the police did yesterday interfered with press freedom brutally," said 45-year-old Kim Yau as she bought a copy.

"All Hong Kongers with a conscience have to support Hong Kong today, support Apple Daily."

Police detained Lai on Monday for suspected collusion with foreign forces after about 200 officers searched the newspaper's offices, collecting 25 boxes of evidence.

On the mainland, the China Daily newspaper wrote in an editorial that Lai's arrest showed "the cost of dancing with the enemy". The Beijing-backed paper added that "justice delayed didn't mean the absence of justice".

Beijing has labelled Lai a "traitor" in the past.

Shares in Lai's media company, Next Digital, which publishes Apple Daily, surged 170% at the open on Tuesday after online pro-democracy forums called on investors to show support.

Mainland-born Lai, who was smuggled into Hong Kong on a fishing boat when he was a penniless 12-year-old, is one of the most prominent democracy activists in the Chinese-ruled city and an ardent critic of Communist Party rule in Beijing.

His arrest comes amid a crackdown against pro-democracy opposition in Hong Kong which has drawn international condemnation and raised fears for the freedoms promised by Beijing when the former British colony returned to China in 1997.

Police arrested 10 people in total on Monday, including other Apple Daily executives and 23-year-old Agnes Chow, one of the former leaders of young activist Joshua Wong's Demosisto group, which disbanded before the new law came into force.

The sweeping security law imposed on June 30 punishes anything Beijing considers secession, subversion, terrorism or collusion with foreign forces with up to life in prison.

U.S. Secretary of State Mike Pompeo on Monday called Lai a "patriot" and said his arrest showed that Beijing had "eviscerated" Hong Kong's freedoms and eroded the rights of its people.

Britain said the arrest was further evidence the security law was a "pretext to silence opposition".

China's embassy in Britain responded by urging London to stop "using freedom of the press as an excuse to discredit" the national security law.

It accused Britain of "supporting anti-China elements seeking to disrupt Hong Kong" and interfering in its internal affairs.