Harvey Milk, the U.S.'s first openly gay rights activist elected into office, has been given honours in a new U.S. postage stamp.

The stamp in his honour was unveiled at a White House ceremony attended by gay rights leaders and various U.S. public officials.

Milk, an elected official and a member of San Francisco's board of supervisors, was slain in office in 1978. Along with the city's mayor, he was gunned down by a political rival.

Albeit able to serve his constituents for only 11 months, his office was known for passing a stringent gay rights ordinance for the city, which included extended employment protections to gays and lesbians in San Francisco.

Mr Milk embodied the best of American ideals, Samantha Power, the U.S. Ambassador to the United Nations, said during the unveiling ceremony.

"He was impatient with excuses. He was intolerant of injustice. He demanded dignity for himself and for all Americans," Power said. "In so doing, he helped to make America more fair, more just, and more equal. In short, Harvey Milk made America more American."

"The tiny ripples that began more than 40 years ago began a tidal wave that swept through the Supreme Court and the nation, liberating gay couples in states around the country," Rep. John Lewis (D-Georgia), a veteran of the civil rights movement, said. "So, thank you, Harvey Milk."

James Wigdel, a USPS spokesman, said stamps bearing Milk's black-and-white image "are selling briskly."

Early in the morning long before the unveiling ceremony began, Mr Wigdel said long lines started to form in front of a U.S. Post Office in San Francisco's Castro District.

May 22 is Harvey Milk Day throughout the state.

"He said his victory signaled a green light to all those who feel disenfranchised, a green light to move forward and that the doors are open to everyone," House Minority Leader Nancy Pelosi (D-San Francisco), recalled Milk's words after he won a seat on the board of supervisors.

"Thanks to Harvey, our society is better forever. Together, we'll finish the work Harvey started."