On the statue's position on the ceremonial island in Waterloo Place, Banksy said: "There was a bit of a gap."
Since it appeared on Wednesday, crowds have gradually increased in size.
"With Banksy, it's a limited time event because it's public art - you don't know how long it's going to be up," said 23-year-old student Ollie Isaac as he joined dozens taking a look.
"I think it's brilliant," he added, saying he thought the statue was a response to "the resurgence of nationalism in the world and this country".
"That suit screams politician," he added.
Teacher Lynette Cloraleigh, 55, came down after a friend posted about the statue on Instagram.
"I like it," she said. "I like where it is. (It's) intriguing how it got here."
Contractors were seen erecting safety barriers around the work on Thursday afternoon.