Hong Kong's legislature passed on Wednesday a controversial immigration bill, which lawyers, diplomats and right groups fear will give authorities unlimited powers to prevent residents and others from entering or leaving the Chinese-ruled city.

"We are facing increasing challenges, especially preventing the number of illegal immigrants from rising and claimants from abusing the system," Security Secretary John Lee said, adding that travel rights remain guaranteed and that the government will introduce subsidiary legislation in the near term.

The government has dismissed those fears as "complete nonsense," saying the legislation, which will come into effect on Aug 1, merely aims to screen illegal immigrants at source amid a backlog of asylum applications and does not affect constitutional rights of free movement.

The assurances, however, come in a climate of mistrust after the increasingly authoritarian path officials have taken the imposition of a sweeping national security law by Beijing last year.

"What is concerning is that in hastily pushing this bill forward, the government has chosen to ignore civil society groups that have flagged legitimate concerns," said Michael Vidler, a lawyer with Vidler & Co Solicitors.

A group of US senators last year estimated at least two dozen US citizens had been prevented from leaving China in recent years and face regular surveillance and harassment by authorities. China denies foreign nationals are under threat of arbitrary detention or exit bans.

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