Politicians, celebrities, and nonprofit leaders—speaking to more than 1,200 people in Washington—today urged Americans to take up President-elect Barack Obama’s call for a new era of community service.

Speakers at the breakfast gathering, which kicked off a national day of service in honor of Martin Luther King Jr., said Mr. Obama’s presidency would give new momentum to the effort to get more people involved in solving the country’s problems.

“We have a champion in the White House,” said Alan Khazei, chief executive of Be the Change, a civic-engagement group in Boston that helped start ServiceNation, a coalition of more than 120 organizations that organized the breakfast.

“We all must double down now and seize this moment,” he said. “Otherwise it may pass us by.”

Mr. Obama asked people to volunteer on the King holiday, but also to make a commitment to work regularly to improve their communities.

Martin Luther King III, the slain civil-rights leader’s son, endorsed that idea at the breakfast. “The goal is to keep that service going through the year,” said Mr. King, chief executive of Realizing the Dream, a community- and youth-development group in Atlanta. He said his father often said that anybody could serve their country—“you don’t have to have a Ph.D. to serve.”

Indeed, Mr. Obama’s endorsement of the national day of service fueled immense interest. The Corporation for National and Community Service, the federal national-service agency, reported that people organized a record number of more than 12,000 projects across the country — double last year’s number.

Arne Duncan, Mr. Obama’s choice for education secretary, represented the incoming administration at the breakfast. He said it was “crucial” that Congress adopt legislation to expand national service programs this year, which Mr. Obama has already pledged to sign.

Mr. Khazei, Mr. King, and others also urged Congress to adopt the Serve America Act—which has been introduced by Sen. Orrin Hatch, Republican of Utah and Sen Edward Kennedy, Democrat of Massachusetts—within the next 100 days.

The legislation would more than triple participation in year-long national-service programs like AmeriCorps, to 250,000, by 2013.

It would also create several new service programs and provide money to help nonprofit groups charities recruit volunteers and spread innovative projects.

ServiceNation and other nonprofit leaders were hoping that legislation would become part of the economic-stimulus plan Congress and the Obama administration will work on over the next few weeks to help pull the nation out of recession. House Democrats proposed a stimulus plan last week that did not include the measure, although it did call for $200-million in new spending on AmeriCorps.

Speakers at the event included Maria Shriver, wife of California’s Republican governor, Arnold Schwarzenegger (who urged other states to follow California’s lead and create a cabinet-level post for volunteerism); and three Democratic members of Congress—John Lewis of Georgia and Doris Matsui and George Miller of California.

Celebrity endorsements came from Demi Moore and her husband, Ashton Kutcher, film actors who showed a Web video they created featuring entertainers pledging to take action to improve the world—for example, by buying a hybrid car, using less plastic, smiling more, or helping to find a cure for Alzheimer’s disease.

Another film actor, Tobey Maguire, said he had started a ServiceNation “ambassador’s council” to recruit Hollywood starts to promote community service.

After the breakfast, several hundred people, including Mr. Maguire, headed to a project to paint and beautify nearby Simon Elementary School. The results will be unveiled on Tuesday night at a live broadcast by MTV, the cable television network, at an official inaugural ball for young people.

So many people wanted to volunteer at the school that organizers had to cut off applications and send people elsewhere. Cathy Cronen, 63, a nurse from suburban Portland, was among those who made it in. In town to visit her daughter during the inauguration festivities, she said she volunteered specifically because she wanted to honor Mr. Obama’s request.

“This is part of what Barack Obama is calling us to do,” she said. And, as the President-elect suggested, she plans to follow up by committing to a regular community-service project—volunteering to tutor or mentor school children.