Pictures posted to the group's website showed protesters carrying signs with slogans like "I ride inside" and "Mitt is Mean!"
The incident has become fodder for Romney's opponents on the campaign trail.
Late last month, David Axelrod, President Obama's chief political consultant, used Twitter to tweak Romney over the incident, tweeting a picture of Obama riding in the presidential limousine with his dog, Bo. He included a veiled swipe at Romney:
"How loving owners transport their dogs," Axelrod wrote.
Newt Gingrich also referenced the incident in a campaign ad last month.
Romney has since defended the decision, saying the dog was comfortable.
"This is a completely air-tight kennel and mounted on the top of our car,” Romney told Fox News. “He climbed up there regularly, enjoyed himself. He was in a kennel at home, a great deal of time as well. We loved the dog. It was where he was comfortable and we had five kids inside the car. My guess is he liked it a lot better in his kennel than he would have liked it inside."