Democratic lawmakers heckled Hong Kong's leader in parliament on Wednesday, slamming him for not speaking out over five missing booksellers feared to have been detained by authorities in mainland China.

The five from Mighty Current publishing house, known for books critical of Beijing, disappeared last year, stoking fears that Chinese control is tightening in semi-autonomous Hong Kong.

The latest to vanish is Lee Bo, 65, last seen in Hong Kong on December 30. Three others went missing in southern China and one in Thailand, all in October.

Hong Kong leader Leung Chun-ying was lambasted as he delivered a key policy speech, with several democratic lawmakers shouting at him before they were removed by security.

"Where is Lee Bo? Why don't you answer Hong Kong people," Labour Party's Lee Cheuk-yan yelled.

Pro-democracy lawmakers, activists and some residents believe the publishers were kidnapped by Chinese mainland authorities, and the disappearances have triggered outrage and protests in recent weeks amid fears freedoms in the city are eroding.

Critics accuse Beijing of trampling on the "one country, two systems" agreement under which Hong Kong was guaranteed civil liberties when it was returned by Britain to China in 1997.

"This is the important matter. Any other policy is meaningless if the 'one country two systems' is finished," Lee said.

Another lawmaker, Raymond Chan, shouted: "Even the basic safety of Hong Kong people can disappear, why aren't you talking about this?"

Dozens of demonstrators also gathered outside the legislative council building where Leung was speaking.

"He does not make Hong Kong people's well-being a priority," activist James Hon told AFP.

"He should immediately take action to rescue the five people from the... bookstore."

Hong Kong officials have said they requested information on the booksellers' whereabouts from Chinese authorities, but have not received an answer.

Jasper Tsang, the parliament's speaker, has also spoken out against the disappearances.

"We have one country, two systems... our way of life is such that no one should disappear suddenly," he told Hong Kong's South China Morning Post on Tuesday.

In his speech, Leung also warned of "obstacles" ahead for the city's economy amid a global economic slowdown, triggered by tumult in China, that hit Hong Kong's trade sector last year.

Residents of the city are increasingly discontent over soaring property prices and lagging wages, taking a further toll on the government's popularity.

Leung's approval rating has plunged to a low of 37.5 percent, according to an opinion poll released by the University of Hong Kong on Tuesday.