WASHINGTON (AP) — First lady Michelle Obama challenged George Washington University graduates at their commencement Sunday to “keep giving” through community service work and to “keep engaging” with the world.

Obama spoke to some 5,000 graduates and their families at the ceremony on the National Mall. She agreed to be their speaker after students, faculty and staff met her challenge to complete 100,000 hours of community service.

“I have one more request to make of you, one more challenge,” Obama said during her speech. “Keep going. Keep giving. Keep engaging.”

Graduates in black robes cheered as Obama spoke with the Capitol behind her. The university said that 163,000 hours had actually been worked. Obama noted many of the accomplishments: improving a Washington school, visiting with veterans, teaching English to refugees and shoveling snow during a record winter snow storm.

But she urged students to continue the work, both in the United States and abroad. She said serving would make “the world safer” and make the students “more competitive.”

WATCH ‘NIGHTLY NEWS’ COVERAGE OF MICHELLE OBAMA’S COMMENCEMENT ADDRESS:

Visit msnbc.com for breaking news, world news, and news about the economy

“So many of today’s challenges are borderless — from the economy to terrorism, to climate change … more than any other generation yours is fully convinced that you’re uniquely equipped to solve those challenges,” said Obama, who spoke for about 30 minutes.

She expressed confidence in the graduates.

“You guys can’t be stopped. You don’t know the meaning of the word can’t. And every time someone’s tried to tell you that you’ve replied what? Oh yes, we can,” Obama said, a refrain President Obama used during his campaign.

Obama said that at her husband’s inauguration, “he pledged to seek a new era of American engagement, and he asked each of us to embrace anew our duties to ourselves, our nation and the world.”

“Now I’m not a president. I’m just a citizen. But as a citizen I’m asking you as graduates of this global institution to seize those responsibilities gladly,” she said. “I’m asking you to play your part.”

Graduates said the first lady’s words moved them and had motivated them during the year.

Gilbert Rein, 21, a graduate from Marlboro, New Jersey, said he had always done volunteer work with his fraternity, but the first lady’s challenge pushed them to do more.

“This year we went above and beyond,” he said.

Rein, who is going to law school, said he plans to do pro bono work to continue to give back.

Saturday’s speech was not the only commencement address the first lady has given this year. Earlier this month she spoke at the University of Arkansas at Pine Bluff, a historically black college. Obama, who graduated from Princeton and Harvard Law School, will also speak next month at the graduation of Anacostia Senior High School in Washington, a public school she visited last year as part of her mentoring program for young women.

During the ceremony, George Washington University president Steven Knapp presented Obama with an honorary Doctor of Public Service degree. Pianist and composer Dave Brubeck and entrepreneur A. James Clark also received honorary degrees at the commencement.

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.