A senior Beijing official overseeing Hong Kong affairs on Monday called on the executive, the legislature and the judiciary to play its own part and complement one another's work, saying it's key for the executive-led political system in the SAR.

Xia Baolong, director of the Hong Kong and Macao Affairs Office, also told a seminar that Hong Kong had faced challenges in implementing the governing mechanism, with attempts by anti-China and external forces to advocate for "separation of powers".

"[They] had hoped to weaken the authority of the chief executive and the SAR government, thereby undermining the authority of the central government as well as resisting and rejecting the power of central authorities. This is absolutely not permitted," Xia was quoted as saying in a statement by his office.

The seminar in Beijing, held by the Chinese Association of Hong Kong and Macao Studies think tank, focused on the executive-led political system, with government officials and politicians attending remotely in the SAR.

In his address, the Beijing official said the key to maintaining and enhancing the governing principle is for the chief executive to take full responsibility in leading the SAR, while aligning with the country's latest five-year plan and being more forward-looking and proactive in pushing for development.

The executive-led system requires active support and cooperation from the legislature and the judiciary, Xia said, adding that the three branches must offer mutual help and not undermine one another.

Chief Executive John Lee, in a social media post, pledged to take up the prime responsibility in leading Hong Kong and to improve the governance efficacy of his administration.

He also vowed to lead his cabinet to deliver on Beijing's hopes, while calling on various sectors of society to play their part in safeguarding the executive-led system.

Xia also outlined five expectations for lawmakers, including paying great attention to their personal conduct and being involved in the government's work.

"[Lawmakers] have to play an important role in the SAR's reform and development, and not be outsiders," he said, and they should stand firm on important matters "against anti-China and external forces in the United States and the West".

Chen Shaobo of the think tank said Xia's expectations for the legislature could be used to assess the performance of Legco members, adding that: "These are important for lawmakers on how to do their job."

Lawmaker Priscilla Leung said Xia called on the legislature to fully support the country's latest five-year plan and urged legislators to prioritise the work of Legco.

"[He] has much higher expectations of the Legislative Council in terms of the direction, professionalism as well as the establishment of our image, being a model for the community, and fully participating in community work," Leung said.

Fellow lawmaker Wu Yingpeng, for his part, said the Beijing official's speech offered a clear course for the development of the city's political system, particularly on the executive-legislative relationship.

Legco president Starry Lee pointed out that lawmakers have arranged a gathering to study key messages from the seminar, which she said has clearly outlined the roles and duties of the new legislature.

She said legislators would put their heads together, perform their constitutional duty and fully support the government.