HONG KONG—Protesters here have dialed up attacks against the Chinese Communist Party ahead of mass demonstrations planned for the 70th anniversary of the People’s Republic of China on Tuesday, renewing momentum for the protest movement by shaping it as a fight against authoritarianism.

During weekend protests marked by the fiercest clashes between demonstrators and police in recent weeks, many participants expressed a vehement rejection of Communist rule in China, which Beijing will celebrate on Tuesday with a grand military...