Image copyright Meredith Blake Image caption The city's mayor raised the transgender flag over Boston's city hall as a controversial bus arrived in the city

The mayor of Boston has backed transgender rights in the city, as a controversial "free speech" bus arrived in Massachusetts.

The bright orange bus criticises what it calls "gender ideology" with the words "you can't change sex" and "it's biology" on its side.

It closely resembles a Spanish bus created by a Catholic group that was forced off the roads by protesters.

The US bus was vandalised at its first stop in New York.

It is sponsored by conservative action groups the National Organization for Marriage, Citizen Go and the International Organization for the Family.

The organisers say it is aimed at promoting "the truth of gender" which they say is "determined by biology rather than by emotions and feelings".

Image copyright Mayor Walsh's office Image caption Boston's mayor flew the transgender flag over city hall

Speaking while the transgender flag was raised over over City Hall, Boston's mayor Marty Walsh said: "We are listening to our transgender community by making the city more inclusive.

"Its important for us to every now and then send a message, to let people know we will not be intimidated by discrimination or harassment.

"When you deny the experience of transgender individuals, you are denying the experience of basic human civil rights - in Boston and Massachusetts we are better than that.

"Together we are going to continue to fight intolerance with love and acceptance, and we will continue to fly this flag with pride in our hearts."

Protesters surrounded the bus at the Massachusetts State House in Boston and in neighbouring Cambridge, near Harvard University.

One protester wrote #HateSpeechBus on the bus in response to the campaign's slogan #FreeSpeechBus.

Image copyright Meredith Blake Image caption The bus has seen protests in New York and Boston, but still plans to go visit four more states

A spokesman for the National Organization for Marriage told the BBC he was "disappointed that people who preach tolerance and acceptance reacted to the bus by lashing out".

The spokesman insisted a significant proportion of the US population felt "uncomfortable" about the prospect of transgender people using public facilities such as changing rooms and bathrooms.

He said the vandalism of the bus showed how "people with alternative point of view on transgender issues are treated", and called for their opinion to be treated with respect.

The group had previously described the incident in New York as a "vicious assault" by "LGBT extremists" but said it was committed to continuing the East Coast tour.

The bus will now travel to Connecticut, Philadelphia and Baltimore before its final stop in Washington DC.