The president of a conservative group in Spain has vowed to fight on after a judge impounded a bus that was due to tour the country protesting transgender ideology.

Ignacio Arsuaga, president of Hazte Oír, accused the judge of “kidnapping” his bus, which had a slogan on the side saying: “Boys have penises, girls have vaginas. Don’t be fooled.”

The bus generated outrage among left wingers after its first day on the streets of Madrid on Monday, resulting in the city council banning it from their roads. Judge José Juan Escalonilla then backed the decision, with Madrid prosecutors now considering whether to investigate it for hate crime.

They say that failure to do so could risk “disturbing the peace and creating a feeling of insecurity or fear among people due to their identity or sexual orientation, and specifically among minors who could be affected by the message.”

However, Mr. Asuaga said his group had the freedom to protest “sexual indoctrination” in schools, adding that the slogan on the side of the bus is based on a “fact of biology”.

“We are going to appeal this unjust decision and file a criminal case against the extreme-left Madrid mayor,” he said.

La Gaceta reports the group now plans to cover the bus with a tarpaulin containing different messages in order to circumvent the ban and continue travelling Friday. They may also charter a second vehicle with the same message.

The group created the campaign in response to posters put up in cities across the north of Spain depicting naked children with genitalia of the opposite sex.

The posters, created by transgender advocacy group Chrysallis, declared: “There are girls with penises and boys with vaginas. It’s as simple as that.”

They were funded by an anonymous American donor who gave €28,000 (£24,000) to help the group spread its message.