U.S. President Barack Obama on Wednesday signed into law a landmark bill allowing gays and lesbians to serve openly in the military.

Obama signed the "Don't Ask Don't Tell Repeal Act of 2010" in a ceremony at the Interior Department, surrounded by approximately 500 attendees including senior military officials and members of Congress.

"This law I'm about to sign will strengthen our national security," the president said.

Obama said that valor and sacrifice in the armed forces are no longer defined by sexual orientation.

The "Don't Ask, Don't Tell" policy, which was introduced as a compromise measure in 1993 by then-President Bill Clinton, prohibited homosexuals from serving openly in the military.

The measure has been a major contentious issue in U.S. politics. Critics including gay rights groups argue that the policy violated the rights of gay military members to free speech and open association.

The signing ceremony was a breakthrough moment for the nation's gay community, the military and for Obama himself. The president had faced pressure from liberals, an important base in his Democratic party, who complained he was not acting swiftly enough.

The military service chiefs must complete implementation plans for lifting the ban on gays before ending the old policy -- and they must certify to lawmakers that it will not damage combat readiness, as critics charge.

Obama promised, however, that "we are not going to be dragging our feet to get this done."

"No longer will tens of thousands of Americans in uniform be asked to live a lie, or look over their shoulder in order to serve the country that they love," Obama said.

Defense Secretary Robert Gates said defense officials will immediately proceed with the planning necessary to "carry out this change carefully and methodically, but purposefully."

Obama hailed the "courage and vision" of Gates and praised Gates and Joint Chiefs of Staff Chairman Adm. Mike Mullen, who advocated changing the 17-year-old policy.