Gay activists rallied in major cities statewide to protest a new law that critics said could nullify legal contracts between same-sex couples.

The state law, which went into effect Thursday, prohibits civil unions, partnership contracts or other arrangements "purporting to bestow the privileges or obligations of marriage."

Critics said it could be used to nullify medical directives, wills, joint bank accounts and other agreements between gay couples.

"(The law) clearly states that gay and lesbian people in this state should not feel welcome," said Dyana Mason, executive director of Equality Virginia (search), the state's largest gay rights organization. "It seeks to strip the only tool that gay and lesbian couples have to protect their families."

Mason spoke at a rally in Richmond on Wednesday that drew more than 400 gay activists and supporters to the Capitol grounds. Simultaneous rallies were held in Norfolk, Charlottesville, Fredericksburg, Fairfax, Staunton and Roanoke, drawing more than 1,000 people.

Virginia Attorney General Jerry Kilgore (search) has said the law provides a needed safeguard for the institution of marriage, and does not deprive anyone of rights to enter into contracts such as wills or other agreements between gay couples.

But Gov. Mark R. Warner (search), a Democrat, issued a statement condemning the law, which he refused to sign.

"This law raises serious constitutional issues and it places Virginia outside the mainstream of other states when it comes to respecting individual liberty," he said.

Kent Willis, executive director of the American Civil Liberties Union (search) of Virginia, said his group was working with others to legally challenge the law. He said it was so vaguely worded that it could be used against heterosexuals of the same sex who enter into legal agreements with each other.

Kilgore has vowed to defend the law if challenged.

The conservative Family Foundation said in a statement that gay rights groups are "willing to frighten and mislead their supporters simply to further their own political agenda."

"All the rallies in the world won't change the fact that this law passed with a bipartisan supermajority of the General Assembly," said Victoria Cobb, spokeswoman for the group.