DENVER — Six gay couples filed a lawsuit Tuesday seeking to overturn Colorado's gay marriage ban, adding to an escalating showdown in the state as its Republican attorney general urges Boulder officials to stop issuing marriage licenses to same-sex couples.

Although Colorado's constitution bans same-sex marriage, a ruling from the 10th U.S. Circuit Court of Appeals in Denver last week regarding a Utah case said states cannot prevent people from marrying based on their gender.

That motivated Boulder County Clerk Hillary Hall to begin issuing marriage licenses, even though the 10th Circuit put its ruling on hold pending an appeal.

The ruling also invigorated attorneys for six gay couples to file a federal lawsuit in Denver asking for an injunction stopping all officials from enforcing Colorado's ban. Voters approved the constitutional gay marriage ban in 2006.

Even if an injunction succeeds, the U.S. Supreme Court is expected to be the final authority on the matter.

"We are standing together today on the side of love," said Kate Burns, who along with partner Sheila Schroeder is among the couples who filed suit.

Meanwhile, Attorney General John Suthers warned Hall she will face unspecified legal action from his office if she doesn't stop issuing marriage licenses to same-sex couples.

Suthers said his office and Boulder County should appeal to the Colorado Supreme Court to determine whether Hall has the authority to issue same-sex marriage licenses.

Hall had asked Suthers if she could have until July 10 to reply to him. Suthers agreed but remained adamant that she stop issuing licenses immediately.

Hall responded that she would continue giving licenses to gay couples. Suthers' spokeswoman Carolyn Tyler had no comment on Hall's decision.

Mari Newman, one of the attorneys filing suit, said the legal action was filed in part to support Hall, whom Newman believes is acting lawfully.

"We are seeking, among other things, to vindicate the right of the Boulder clerk and recorder," Newman said.

After the 10th Circuit ruling, two of the couples in the lawsuit sought marriage licenses in Denver and Jefferson counties and were denied.

The lawsuit alleges the state's same-sex marriage ban violates due-process and equal-protection rights guaranteed in the U.S. Constitution.

"Colorado law unlawfully denies the issuance of marriage licenses, and refuses to recognize the marriages of certain couples, based solely on the sex of the persons in the marriage union," the lawsuit says.

Although some of the couples in the lawsuit have civil unions in Colorado, the lawsuit calls them unequal and an inadequate substitute to marriage.