(CNN) -- Events around the United States honored the Rev. Martin Luther King Jr. as the nation marked the 25th anniversary of the holiday in honor of one of the country's most prominent civil rights leaders.
President Barack Obama and first lady Michelle Obama marked the day by participating in a service project in Washington.
"This is part of what America is all about," the president said. "After a painful week where so many of us were focused on tragedy, it's good for us to remind ourselves of what this country's all about."
Obama's remarks referenced the January 8 shooting in Tucson, Arizona, that left six people dead and 13 injured.
The project was sponsored by Greater DC Cares, a philanthropic group that organized some 2,000 volunteers to participate in public service works, including student mentoring and community painting.
"Dr. King obviously had a dream of justice and equality in our society, but he also had a dream of service," Obama said in a written statement. "And that's what this program is all about and that's what these participants are all about."
A federal holiday to honor King, who was assassinated in April 1968, was first observed in 1986. In 1994, Congress also designated it a national day of service.
A video posted on mlkday.gov, the U.S. government website dedicated to the day, quotes King: "He who is greatest among you shall be a servant. That's the new definition of greatness. ... By giving that definition of greatness, it means that everybody can be great, because everybody can serve."
The site calls on Americans to honor King by "by pledging to take at least 25 actions during 2011 to make a difference for others and strengthen our communities."
"Racism and bigotry are no longer tolerated in this country as they once were, and the doors of opportunity stand open to children of every background," Democratic National Committee chairman Tim Kaine said in a statement. "But we remain an imperfect nation. We must continue to strive to better ourselves and our country by upholding timeless values that Dr. King exemplified: truth, courage, compassion and justice."
"This holiday should not be one simply of quiet remembrance but of active service to those in our community who need it most, those who Dr. King would have regarded with compassion and treated with charity," said the committee's vice chair, Donna Brazile.
The Republican National Committee also released a statement, in which Chairman Reince Priebus said King was "a leader who was committed to individual freedom and allowing every citizen the opportunity to achieve their dreams.
"America was founded on the principles of freedom and liberty for all, and the Republican Party will continue working towards ensuring every citizen in this country has an equal chance of living the American dream," Priebus said.
The NAACP invited Americans to post on its website and finish the sentence, "I have a dream ... ." King used the phrase in his famous 1963 speech in Washington.
"Whether it's a small-town girl inspired by Black History Month or a prisoner seeking redemption through rehabilitation, the core of Dr. King's message transcends boundaries and permeates the spirit of people today just as strongly as it did 48 years ago," NAACP President Ben Jealous said in a statement. "On this historic day, tell us about your dream so we can remember that America is a nation where hopes and dreams are realized."
Vice President Joe Biden and numerous Cabinet secretaries and administration officials also participated in Monday's service events, the White House said.
King's son, Martin Luther King III, delivered the keynote address at a memorial service at Atlanta's Ebenezer Baptist Church, while the Rev. Al Sharpton's National Action Network said it planned to mark the day with a breakfast in Washington and a public policy forum in New York.
Meanwhile, Mississippi Gov. Haley Barbour announced celebrations scheduled to begin May 22 commemorating the 50th anniversary of the "Freedom Rides," a term used by civil rights activists who in 1961 tested a U.S. Supreme Court decision -- which made discrimination against interstate travelers illegal -- by riding buses throughout the segregated U.S. south.
One hundred twenty-five of the original Freedom Riders are planning to attend events in Mississippi, according to a written statement from the governor's office.
"On the holiday celebrating Dr. Martin Luther King's birth and life of service, it is altogether fitting and proper we formally announce another historic celebration," Barbour said in the statement. "The 100-plus Freedom Riders participating in the 2011 celebration will find Mississippi an enormously changed state as to race relations."
But a Colorado school board member and radio station owner stirred controversy, broadcasting the reading of an anonymous letter he said he received three years ago which described King as a "sexual degenerate" and "an America-hating communist," according to CNN affiliate KMGH.
Greeley, Colorado, resident Brett Reece said he is broadcasting the letter twice a day because he believes in "the general message of what this letter is about," he said.
The broadcasts have left some residents concerned about the negative image they say Reece is portraying about their town.
"In my opinion, I think Greeley is a great town," said resident Angela Mills. "I think we have people like this that represent it in such a poor way that make it look bad."
Anti-Defamation League Regional Director Scott Levin blamed Reece for "spreading divisive and hateful misinformation," calling the radio broadcaster "reckless and irresponsible," according to a league written statement.
Meanwhile, Interior Secretary Ken Salazar noted that a memorial in King's honor is under construction on the National Mall in Washington.
"When completed later this year, the memorial will serve to remind us of Dr. King's hope, sense of justice, and quest for equality," he said in a statement released Sunday.