HONG KONG—Hong Kong pro-democracy legislators heckled a top Chinese official on Monday as he sought to explain Beijing's position on how the city should elect its leader, while police and demonstrators scuffled outside the venue.

Dozens of pro-democracy politicians disrupted remarks by Li Fei, deputy secretary-general of the Standing Committee of the National People's Congress, China's rubber-stamp parliament, which decreed on Sunday that candidates for Hong Kong's top leadership post must be approved by a committee heavily...