BOSTON -- When Ethan St. Pierre decided in 2001 to begin a public transformation from woman to man, he said the security company he worked for at first supported his decision.

Then his features began looking more like a man's.

"Once they saw the changes that my body was making they decided that I could no longer do my job," said St. Pierre, 47, a transgender man living in Haverhill, Mass. "They started taking my responsibilities away from me one at a time until finally they told me that I was no longer welcome."

Supporters of a transgender rights bill making its way through the Massachusetts Statehouse say their goal is to give transgender people like St. Pierre legal protections at work, in public accommodations and in housing.

The bill would accomplish that by adding "gender identity or expression" to a list of protected categories in the state's civil rights and hate crime laws. "Gender identity" refers to an inner sense of being male or female. "Gender expression" refers to the expression of that feeling in clothing, make-up and speech.

The bill is stirring fierce opposition from critics who say it would lead to a breakdown in privacy in restrooms, locker rooms and other single-gender facilities. They also content it would open women's bathrooms to sexual predators.

The Massachusetts Family Institute has begun running radio ads warning mothers that they may no longer want to let their young daughters use public restrooms because "Beacon Hill is about to make it legal for men to use women's bathrooms."

Kris Mineau, president of the institute, said the bill would open up all gyms, showers, restrooms and shelters to anyone of any gender because there's no clear way to determine if an individual is transgender.

"There's no restrictions in this bill as to what opposite gender can use the facility. We're not going to be able to police this," he said. "There's no pre-requirement of surgery or appearance or anything else."

Mineau said the bill would make it easier for the thousands of registered sex offenders in Massachusetts to gain access to children and women in public restrooms by claiming they are transgender.

He said transgender people suffer from a mental disorder and need psychiatric help, not new laws.

"These people need treatment. They need help, we agree. But we don't need to establish public policy that just accentuates this issue, this malady," he said.

Gov. Deval Patrick supports the bill, calling it a "a very straightforward question of human and civil rights."

He also dismissed arguments that the bill might make it easier for sex offenders to lurk in women's bathrooms.

"Somehow we manage at home with bathrooms that don't have 'men' and 'women' on them. And we can probably figure that out on public spaces, too," Patrick said.

More than half of the state's lawmakers have signed on as co-sponsors of the bill.

Hundreds of supporters and opponents of the measure packed a Statehouse auditorium Tuesday to offer testimony on the bill.

Timothy Tracey, a lawyer with the conservative Arizona-based Alliance Defense Fund, told members of the Committee on the Judiciary that the bill infringes on the religious rights of those who believe that men and women are different.

"The First Amendment mandates that no individual should be required to affirm, in act, word, or deed, that a man is a woman, or a woman is a man, against their sincerely held religious beliefs," Tracey said. "Yet this is precisely what (the bill) will do."

Massachusetts Rep. Carl Sciortino, the bill's sponsor along with state Sen. Benjamin Downing, D-Pittsfield, said the bill is designed to expand the state's civil rights laws and is not a threat to public safety.

"What it allows for is that every person, including transgender people, can use facilities that are consistent with their gender identity in a safe and private manner," the Medford Democrat said.

"Anyone that uses a facility to commit a crime or does something indecent can be prosecuted under current laws and this bill does nothing to change that," he added.

Sciortino said he anticipates a debate and vote on the bill in the Legislature by the end of the year. He said a dozen states already prohibit discrimination against people based on whether they identify themselves as male or female.