DENVER - Nine gay Colorado couples today filed a civil rights lawsuit in Denver District Court seeking to overturn the state's ban on same-sex marriage.

"Colorado law creates two classes of citizens: those free to marry the person they love, and those denied that fundamental right. Same-sex couples in Colorado are relegated to a second-class level of citizenship that denies their relationships the full panoply of rights enjoyed by married opposite-sex couples," the lawsuit says. "Plaintiffs bring this action to challenge the constitutionality of Colorado's laws that prohibit same-sex couples from marrying and that refuse to recognize the marriages of same-sex couples lawfully entered into in other jurisdictions."

Colorado permits civil unions but a 2006 voter approved referendum - Amendment 43 - changed the state constitution to define marriage as between one man and one woman. In 2013, the state passed the Colorado Civil Union Act that provides gay couples with rights similar married couples, but stopped short of granting full marriage rights.

Four of the nine couples bringing today's lawsuit were married in other states.

"In their home state of Colorado, however, their valid marriages are reduced to second-class and unequal civil unions, which do not afford them the same rights, protections and security as marriage," according to the suit filed by the Denver law firm of Reilly Pozner.

A spokesman for Reilly Pozner said attorneys and plaintiffs involved in the case were not available for interviews.

The lawsuit names Colorado Gov. John Hickenlooper as a defendant, along with Debra Johnson, the Clerk and Recorder for the City and County of Denver.

"This litigation does not come as a surprise and as a matter of constitutional law we appreciate that the courts will take it up," Hickenlooper said in a statement. "On the underlying question of equal rights, we believe Colorado made a step forward when we passed bipartisan civil unions legislation last year."