A large bus protesting the rise of gender ideology is touring Spain, infuriating left wing campaigners.

The bus, which is emblazoned with the slogan “Boys have penises. Girls have vaginas. Don’t be fooled”, completed a tour of Madrid on Monday and is scheduled to visit major cities across the north and east of the country over the next week.

El Mundo reports that conservative group HazteOír.org paid for the bus in a bid to combat a poster campaign by trans activists depicting naked children with genitalia of the opposite sex. The slogan on the vehicle also declares: “If you are born a man, you are a man. If you are a woman, you will still be one.”

“Our objective is to denounce laws of sexual indoctrination that various regions have passed to promote sexual diversity among minors, threatening freedom of education, and the right of parents to educate their children [as they see fit],” the organisation said.

The bus is also designed to promote the group’s book Did You Know What They Want to Teach Your Child at School? which shows the “reality” of what gender diversity laws really mean.

On its first day out in Madrid, the bus generated outrage among leftwingers, with transgender advocacy group Platform for Trans Rights calling for the state prosecutor to intervene and stop the “transphobic” campaign.

Mar Cambrollé, the group’s president, called the campaign an “unpunished act of hatred and phobia directed at transsexual people” and a “new attack against diversity and human rights”.

The campaign comes after the group Chrysallis put up posters across northern cities including Bilbao, San Sebastián, and Pamplona showing children with genitalia of the opposite sex above the slogan: There are girls with penises and boys with vulvas. It’s as simple as that.”

The posters were funded by an anonymous American donor, who gave €28,000 to help the group spread its message.