HONG KONG — On a holiday meant to celebrate the birth of China’s communist republic, Hong Kong residents instead swarmed the streets Wednesday to protest Beijing’s iron grip over their government and demand democratic reforms.

The massive crowds appeared to be one of the biggest amid a week of demonstrations that have brought large parts of the city to a standstill.

Throughout Hong Kong, it was a day of jarring images and symbolism as authorities tried to carry on with National Day celebrations only to have protesters respond with acts of emotional though peaceful defiance.

Hong Kong’s top official, Leung Chun-ying, began the morning sharing a champagne toast with other Chinese officials while demonstrators nearby booed and jeered. Then, as China’s national anthem played, a group of student protesters turned their backs on a Chinese flag being raised and silently crossed their arms above their heads in a gesture of objection to the Chinese government.

A ceremony planned later to honor Hong Kong war heroes was canceled. And an afternoon event at Victoria Park drew sparse attendance, even as massive crowds began converging near government headquarters, the heart of demonstrations in recent days. Protesters warned that pressures could intensify if authorities ignore their demands, which include Leung’s resignation and forcing Beijing to back down on plans to vet candidates in Hong Kong elections.