"The Basic Law sets out clearly the principle of the separation of powers between the legislature, the executive and the judiciary, and in quite specific terms, the different roles of the three institutions," Ma said.

Worries about Beijing's attitude arose in 2008 when Xi Jinping , then Vice-President, said the three branches of government should give each other mutual support and understanding. He was speaking during a visit to Hong Kong.

This time, Zhang went further, suggesting the chief executive was above the three branches - an unprecedented remark from a Beijing official.

Albert Chen Hung-yee, a legal scholar and member of the Beijing-appointed Basic Law Committee, said the chief executive would enjoy only a "ceremonial" position above the three institutions.

In reality, he said, the role did not entail criminal immunity and "any chief executive would be treated equal to anyone else under the law".

Elsie Leung Oi-sie, the committee's vice-chairwoman and a former justice secretary, said the chief executive's power was clearly defined and restrained under the Basic Law.

It was those restraints - which include stipulating that a chief executive must resign in the face of repeated Legislative Council objections to a key bill - that invalidated Zhang's assertion, said Democratic Party founding chairman and former Basic Law drafter Martin Lee Chu-ming.

While the phrase "separation of powers" was not written into the Basic Law, articles within it implied it. Article 64 states the Hong Kong government "must abide by the law and be accountable to the Legislative Council of the region".

Article 52 states that the chief executive must resign if the Legislative Council is dissolved after the leader refuses to sign a bill passed by the council and the new Legco again passes it, or when the council is dissolved because it refuses to pass the budget or an important bill and once more blocks the legislation.