Answer: Hyperbole
Explanation: Hyperbole is a figure of speech that involves exaggerating or overstating something for emphasis or effect. In this case, the speaker is exaggerating the length of the procession by saying that it is "longer than the Yangtze River," which is one of the longest rivers in the world. This is not meant to be taken literally but rather is used to convey the idea that the procession was very long and impressive.