Let \( x \) represent the total number of people in attendance at the zoo on Monday. According to the problem, 65 percent of the people were children, and the number of children is given as 520. We can express this relationship with the following equation:
\[ 0.65x = 520 \]
To find \( x \), we can solve the equation by dividing both sides by 0.65:
\[ x = \frac{520}{0.65} \]
Now, performing the division:
\[ x = 800 \]
Thus, the total number of people in attendance at the zoo on Monday is \( \boxed{800} \).