Police in Madrid have banned a bus emblazoned with the anti-transgender message: "Boys have penises, girls have vulvas. Do not be fooled."

Catholic group Hazte Oir, which translates as "Make Yourself Heard", was about to embark on a nationwide tour in their bright orange bus when Spanish authorities impounded the vehicle, branding the tour "a hate campaign based on intolerance".

One side of the bus read: "If you are born a man, you are a man. If you are a woman, you will continue to be one."

The Catholic group is thought to have launched the campaign in response to posters put up in northern Spain by transgender rights group Chrysallis which read: "There are girls with penises and boys with vulvas. It's as simple as that."

Hazte Oir launched a petition to ban the posters, but did not manage to gather enough signatures.

In a written statement the group said that the objective of the nationwide bus tour was 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 bus, which was spotted being driven around Madrid on Monday (27 February), sparked outrage among activists and politicians.

Purificacion Causapie, a spokeswoman for the socialist party, said the bus was "contrary to the dignity and rights of transsexual children", and urged the mayor to ensure that Madrid remained "a city free of discrimination, violence and attacks on minors".

Madrid's mayor Manuela Carmena said she wanted the bus to leave the city "as soon as possible".

Members of Hazte Oir said the ban was illegal and that they plan to acquire a new bus.

"We are going to appeal this unjust decision and file a criminal case against the extreme-left Madrid mayor," Hazte Oir's president Ignacio Arsuaga told the BBC.

"The Madrid City Council kidnapped the bus this morning, with no legal order from any court. This is illegal, and our lawyers are preparing a complaint so that our right to freedom of expression is restored."