An advertising campaign running on City of Regina buses has caught the attention of people who feel the city should not allow controversial topics to appear on public transit, but the city says a Supreme Court of Canada ruling leaves it little choice in the matter.

The ads, from Regina Pro-Life, protest abortion.

An image in the campaign of a baby in the womb has prompted some people to protest the use of the ad on a bus.

"It's right there on city property," Shannon Tessier told CBC News Tuesday. "It's disturbing, it's disgusting and it shouldn't be anywhere where someone under the age of, say, 15 or 16 can see it."

She said the combination of the image and the word abortion in accompanying text makes the ad unacceptable to her.

"Obviously if you've had an abortion and you see a picture of that anywhere on the back of a bus or something like that, it's not something that you want to be reminded of that you've done."

Tessier created a group on the internet social network Facebook, looking for others to support her call to the city to remove the ad.

So far, the page has attracted 150 supporters.

The group behind the ad campaign says the image is not of an aborted fetus.

Bob Walldigger said the image is typical of what might be in a school textbook.

"I think people don't want to think about the unborn as a human being," Walldigger said. "That particular ad is so in-your-face that this is an unborn baby."

In the past, the City of Regina has enforced a policy of not allowing any advertising on buses that might generate controversy.

However, a March 2009 ruling by the Supreme Court of Canada on political ads on buses found that limits on advertising violated the right to free speech.

"This is something the community is going to have to get used to because there isn't a lot we can do about it," Jeff Bohach, a City of Regina spokesman, told CBC News.

While controversial topics can be the subject of bus ads, the city can still stop advertising that is illegal, or promotes racism or sexism.