WASHINGTON -- President-elect Barack Obama made no apologies Thursday for asking evangelical pastor Rick Warren to give the invocation at his inauguration, a move that infuriated gays angry at Warren's support for California's Proposition 8 banning gay marriage.

Obama said he and Warren don't agree on everything, but he's collected a group of people to appear at his inauguration who share a variety of viewpoints.

Repeating a line from his campaign, Obama said, "We have to disagree without being disagreeable and then focus on those things that we hold in common as Americans."

Obama noted that he was invited to speak at Warren's Saddleback Church despite Warren knowing that Obama supports gay and lesbian rights, and  Dr. Joseph Lowery will give the benediction at the inaugural ceremony. Lowery holds "deeply contrasting views" with Warren on gay rights issues, Obama said.

Still, gay rights groups say they are appalled at the selection of Warren, whose church is 22,000 strong. Human Rights Campaign president Joe Solomnese sent Obama a letter asking him to reconsider what they say is a show of "disrespect" and a "genuine blow" to the gay community.

The invitation to Warren has "tarnished the view that gay, lesbian, bisexual and transgender Americans have a place" at Obama's table, reads the letter.

"[W]e feel a deep level of disrespect when one of architects and promoters of an anti-gay agenda is given the prominence and the pulpit of your historic nomination," Solomnese wrote.

Andrew Sullivan wrote on the Atlantic Web site, "[It's] shrewd politics, but if anyone is under any illusion that Obama is interested in advancing gay equality, they should probably sober up now."

Warren is joining several luminaries at Obama's swearing-in on Jan. 20. His series of "Purpose Driven Life" books and lectures have made him a bestseller and his church among the largest in the country.

Warren said in a written statement issued Thursday that he and Obama are committed to "civility in America."

"I commend President-elect Obama for his courage to willingly take enormous heat from his base by inviting someone like me, with whom he doesn't agree on every issue, to offer the Invocation at his historic Inaugural ceremony," Warren said.

Warren was not involved in the pastoral leadership that led the campaign in support of Prop 8, although a few days before the election he did release a video and message about his perspective at the request of his parishioners, said Larry Ross, media relations director for the pastor.

Warren "took the biblical perspective on marriage" as a bond between one man and one woman, Ross told FOXNews.com. He said that definition is consistent with Warren's ministry and 5,000 years of history as well as one embraced by every religion.

Ross added that the pastor's view is basically "about being for the biblical definition of marriage, not being against anything" else.

But People For the American Way President Kathryn Kolbert said Warren's support for Prop 8 should have blocked his invitation.

"It's a huge mistake," said California gay rights activist Rick Jacobs, who chairs the state's Courage Campaign. "He's really the wrong person to lead the president into office."

Washington Blade editor, Kevin Naff, called the selection "Obama's first big mistake."

A popular figure among evangelicals, Warren remained publicly neutral during the presidential campaign. He invited both Obama and his Republican rival John McCain to his Saddleback Church in Orange County for a forum on faith and public service.

As many as 4 million visitors are expected to be on hand when Obama takes the noontime oath from Chief Justice John Roberts on the steps of the Capitol. The Obama transition team released the schedule of events on Wednesday.

Some 4,000 local police, 4,000 police from around the country and security agents from other government agencies will be on hand, taking direction from the Secret Service. About 7,500 active duty military and 4,000 National Guard troops also will participate. That includes a contingent on alert to respond to a chemical attack.

A "big chunk" of active and guard units will perform ceremonial work involving parades, reviews and honor guards, the U.S. commander in charge of domestic defense said Wednesday.

Planners are working under the assumption a terrorist or rogue element might try to interrupt the event, said Gen. Gene Renuart, head of the U.S. Northern Command. "So it's prudent for us to plan for the possibility of that kind of event, and to be prepared either to deter it or to respond to it," he said in a session with defense writers.

Also Wednesday, officials announced the list of participants for the inauguration.

The program is to feature poet Elizabeth Alexander; Lowery, a veteran civil rights leader who co-founded the Southern Christian Leadership Conference alongside Martin Luther King Jr.; the U.S. Marine and Navy bands; and the San Francisco Boys Chorus and the San Francisco Girls Chorus.

"So it's prudent for us to plan for the possibility of that kind of event, and to be prepared either to deter it or to respond to it," he said.

Sen. Dianne Feinstein, D-Calif., chairman of the Joint Congressional Committee on Inaugural Ceremonies, said the day would be "an event of historic proportion."

"It is appropriate that the program will include some of the world's most gifted artists from a wide range of backgrounds and genres," she said.

Cellist Yo-Yo Ma, violinist Itzhak Perlman, pianist Gabriela Montero and clarinetist Anthony McGill will perform a new work composed by John Williams, who also provided music for Obama's election night rally in Chicago's Grant Park. The committee did not release a title for the work by Williams, who is best known for his film scores such as "Star Wars" and "Jaws."

Vice President-elect Joe Biden will take his oath from Justice John Paul Stevens.

Others on the schedule were a nod to Obama's election as the country's first black president.

Franklin, a living legend with 21 Grammies, performed for President Bill Clinton in 1993, but this would be her first Inauguration. During a Labor Day weekend rally in Detroit, Obama sang a bit of Franklin's "Chain of Fools" to her.

Alexander, a 2005 Pulitzer Prize finalist and Yale University professor, centers her poems and essays on race relations and social movements.

She is only the fourth poet to have a speaking role at a presidential Inauguration. Robert Frost, who was 86 at the time, wrote a poem for Kennedy's inaugural in 1961 but couldn't make out the words of the poem in the sun's glare. Instead, he recited an earlier work. Clinton chose Maya Angelou to write a poem for his first inaugural in 1993, and Miller Williams read "Of History and Hope" at his second inaugural.

The Associated Press contributed to this report.