Hong Kong police can now demand phone or computer passwords from those who are suspected of breaching the wide-ranging National Security Law (NSL).
Those who refuse could face up to a year in jail and a fine of up to HK$100,000 ($12,700; £9,600), and individuals who provide "false or misleading information" could face up to three years in jail.
It comes as part of new amendments to a bylaw under the NSL that the government gazetted on Monday.
The NSL was introduced in Hong Kong in 2020, in wake of massive pro-democracy protests the year before. Authorities say the laws, which target acts like terrorism and secession, are necessary for stability - but critics say they are tools to quash dissent.
The new amendments also give customs officials the power to seize items that they deem to "have seditious intention".
Monday's amendments ensure that "activities endangering national security can be effectively prevented, suppressed and punished, and at the same time the lawful rights and interests of individuals and organisations are adequately protected", Hong Kong authorities said on Monday.
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The city has seen the arrests of hundreds of protesters, activists and former opposition lawmakers since the introduction of the NSL.
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