BOSTON >> Anti-transgender activists are being met with protests as they drive through cities in the northeastern United States in a big, orange bus emblazoned with the words “boys are boys” and “girls are girls.”

The “Free Speech Bus” parked in front of the Massachusetts State House in Boston on Thursday morning, drawing more than two dozen protesters holding signs and chanting, among other things, “No hate. No fear. Trans people are welcome here.”

Democratic Mayor Marty Walsh, surrounded by dozens of supporters, raised a flag recognizing the transgender community after the bus briefly stopped in front of City Hall.

Gregory Mertz, U.S. director of CitizenGO, the Madrid, Spain-based group that’s behind the bus tours, said organizers are pushing back against laws and policies accommodating transgender people.

“There’s an agenda and movement that’s saying it’s OK for a boy to be a girl and that you can use whichever restroom you want,” he said. “We think that’s very harmful.”

The bus’s message is simply stating the “biological reality” that humans are “binary, sexually complementary creatures,” said Joseph Grabowski, a spokesman for the National Organization for Marriage who was among a handful of supporters riding on the bus.

The full text splashed across the bus’s exterior reads: “It’s Biology: Boys are boys . and always will be. Girls are girls . and always will be. You can’t change sex. Respect for all.”

Protesters said the message is overly simplistic.

“It assumes that our identities are the sum of what’s between our legs,” said Michelle Tat, a transgender woman from Boston helping lead the chants with a bright pink megaphone. “I’d argue that it’s more about our lived experiences and our genders. Biology gives us what we are born with, but it doesn’t make us who we are.”

The bus message may appear benign, but it only serves to fuel rising hatred and violence toward the lesbian, gay, bi-sexual and transgender community, said Mason Dunn, executive director of the Massachusetts Transgender Political Coalition, which helped coordinate Thursday’s protests.

“Words, in this setting, are violence,” he said. “We’re concerned about the health and wellness of our community.”

Grabowski said there was nothing inherently violent about the bus’s message.

“People have the right to live their lives the way they want,” he said. “But they don’t have the right to impose their values and beliefs on others, which is unfortunately what a lot of sexual orientation and gender identity non-discrimination laws do.”

The bus also stopped by Harvard University in nearby Cambridge on Thursday. It’s slated to head south in the coming days, making stops in New Haven, Connecticut, Philadelphia and Baltimore before arriving in Washington, D.C., on April 3.

The campaign was launched in Spain earlier this month with a more direct message that, in Spanish, reads: “Boys have penises, girls have vulvas. Do not be fooled. If you are born a man, you are a man. If you are a woman, you will continue to be one.”

That campaign was in response to pro-transgender posters in Spain that depicted four naked children — including a boy with female genitalia and a girl with a male sex organ — holding hands. Underneath the image was a phrase in Spanish that read: “There are girls with penises and boys with vaginas. It’s that simple.”

The U.S. tour initially launched in New York City last week, but the bus was taken out of commission after it was vandalized outside the United Nations building.

Bus organizers say they won’t be holding press conferences, rallies or confronting counter protesters as they stop in front of government buildings and colleges, largely because of that New York incident.

“We’re calling for a healthier conversation. Let people be respected and heard,” Grabowski said. “A lot of people are afraid to speak out. We want them to be able to stand up and know that we’ve got their backs.”