The United States and Britain have voiced dismay over Hong Kong's arrest Saturday of leading democracy advocates, urging Beijing and Hong Kong authorities to observe the rule of law.

At least 15 pro-democracy activists including former lawmakers and a media mogul were rounded up for allegedly organizing and joining unauthorized protests during last year's monthslong anti-government demonstrations.

(Pro-democracy supporters hold banners and shout slogans outside of the Western District police station in Hong Kong after at least 14 pro-democracy veterans and supporters being arrested in a sweeping operation on April 18, 2020 in Hong Kong, China.)[Getty/Kyodo]

U.S. Secretary of State Mike Pompeo issued a statement condemning the arrests as "deeply concerning."

"Beijing and its representatives in Hong Kong continue to take actions inconsistent with commitments made under the Sino-British Joint Declaration that include transparency, the rule of law, and guarantees that Hong Kong will continue to 'enjoy a high degree of autonomy,'" he said.

U.S. Attorney General William Barr separately called the arrests an "assault on the rule of law and the liberty of the people of Hong Kong.

"These actions -- along with its malign influence activity and industrial espionage here in the United States -- demonstrate once again that the Chinese Communist Party cannot be trusted," he said.

Short of a condemnation, Britain's Foreign and Commonwealth Office stated its concern over the mass arrests and called for a fair and transparent judicial process to be taken against the arrested.

"The right to peaceful protest is fundamental to Hong Kong's way of life and as such is protected in both the Joint Declaration and the Basic Law. It is essential that any protests are conducted peacefully, and that the authorities avoid actions that inflame tensions," an office spokesman said.

Foreign legal communities and even the United Nations have also expressed concern at the clampdown, urging the authorities to drop the charges and stop "politicized and targeted prosecutions."

"Detention &criminal prosecution must not be used to target &intimidate peaceful protesters," tweeted Clement Voule, the U.N. Special Rapporteur on the rights to freedom of peaceful assembly and of association.

Among those detained Saturday are veteran democrats Martin Lee and Margaret Ng, both barristers and former lawmakers, as well as Jimmy Lai, founder of the anti-government Apple Daily newspaper.

The police said the allegations relate to three unauthorized march protests held between August and October, when the anti-government demonstrations, which began in June in opposition to an extradition bill, were in full swing.

The Commissioner's Office of China's Foreign Ministry in Hong Kong on Sunday pushed back against the accusations, saying the Hong Kong police acted in accordance with the law and that foreign states have no right to interfere in Hong Kong's affairs.

"Britain's foreign office spokesman ignored the facts...(that) anti-China members are using the disguise of 'peaceful protest' for 'illegal assembly.' This is completely wrong," a commission spokesman said in a statement.