In a sign of how seriously it views the situation with the lawmakers, China is taking the extraordinary step of interpreting a clause in the Basic Law in such a way that is expected to make it impossible for Ms. Yau and Mr. Leung to retake their oaths and formally assume office.

A decision may come on Monday. It would be only the second time since 1997 that the National People’s Congress had intervened in Hong Kong without being asked by the territory’s government or its highest court.

Such an intervention into a sophisticated legal system inherited from the British and based on hundred of years of legal precedents has alarmed many people in Hong Kong.

China has the right to issue interpretations of the Basic Law, but the Hong Kong Bar Association said on Wednesday that a decision by Beijing would “deal a severe blow” to the judicial independence of Hong Kong’s courts, which are adjudicating Ms. Yau and Mr. Leung’s case.

Many fear that the move will further undermine the principle of “one country, two systems,” which has given the city considerable autonomy.

Beijing’s impending move has galvanized the large coalition of protesters who captured the world’s attention during their 79-day occupation of major thoroughfares in Hong Kong in late 2014. They fear that the Communist Party will not only invalidate the elections of Ms. Yau and Mr. Leung, but also move against other major figures of the protests who were voted into office in September.

“When Hong Kong’s Basic Law can be changed at the Communist Party’s will, what does that say about Hong Kong’s future?” said Joshua Wong, 20, the most prominent leader of the 2014 protests.