US President Barack Obama presents the Presidential Medal of Freedom to former US President Bill Clinton at a ceremony in the East Room of the White House in Washington, November 20, 2013. [Photo/Agencies]

WASHINGTON - President Barack Obama opened a day of tributes to former President John F. Kennedy on Wednesday by bestowing the Presidential Medal of Freedom on prominent Americans, 50 years after Kennedy was assassinated weeks short of the medal's first award ceremony.

Obama presented the medal - the highest award the US gives a civilian - to entertainer Oprah Winfrey, former President Bill Clinton, and leaders from the worlds of sports, entertainment, science and public service. Hillary Rodham Clinton and Steven Spielberg were among those gathered in the East Room of the White House to watch the ceremony.

"Today, we salute fierce competitors who became true champions," Obama said, pausing to speak in personal terms about each of the recipients and their contributions to society.

The ceremony marks the start of a day honoring Kennedy's legacy two days before the nation pauses to remember the 50th anniversary of his assassination. Later, Obama will pay a visit to the eternal flame that marks John F. Kennedy's gravesite. He'll be joined by first lady Michelle Obama and the Clintons at the ceremony at Arlington National Cemetery.

Turning to the former White House inhabitant, Obama said that Bill Clinton's presidency had been only the start of his work to improve the world, crediting his post-presidency humanitarian works as helping to save or improve the lives of hundreds of millions around the world.

"I'm grateful, Bill, as well, for the advice and counsel that you've offered me, on and off the golf course," Obama said to chuckles. "And most importantly, for your lifesaving work around the world, which represents what's the very best in America."

Obama said the late Sally Ride, the first American woman in space, didn't just break the stratospheric glass ceiling, "she blasted right through it."

"Young girls need to see role models, she said. You can't be what you can't see," Obama said. "Today our daughters, including Malia and Sasha, can set their sights a little bit higher because Sally Ride showed them the way."

Kennedy established the modern version of the medal, but was assassinated by Lee Harvey Oswald in Dallas on Nov. 22, 1963, two weeks before he planned to honor the inaugural group of recipients. Hundreds of noteworthy figures have since received the medal.

As a teenager, Bill Clinton shook hands with Kennedy the summer before the assassination when he and other high school students in the Boys Nation program went to Washington.

In the evening, Obama plans a speech on Kennedy's legacy of service at a dinner at the Smithsonian's National Museum of American History attended by current and past medal recipients, including baseball's Hank Aaron, astronaut Edwin "Buzz" Aldrin, singer Aretha Franklin, former Federal Reserve Chairman Alan Greenspan, activist Jesse Jackson and former Secretary of State Henry Kissinger.

Kennedy's grandson, Jack Schlossberg, is to introduce Obama at the dinner. Other Kennedy relatives are expected to attend, including Robert Kennedy's daughter Kathleen Kennedy Townsend and former diplomat Jean Kennedy Smith, a former medal recipient and John F. Kennedy's only surviving sibling.

Friday marks 50 years to the day since Kennedy was slain while riding in an open car in a motorcade during a visit to Dallas. Obama plans to meet privately at the White House on Friday with leaders and volunteers from the Peace Corps program, also established by Kennedy.

The Clintons' presence at the eternal flame where Kennedy is buried is sure to spark speculation about whether Obama has a favorite in the 2016 race to succeed him.

For Hillary Rodham Clinton, his former secretary of state, her every move is scrutinized for signs of whether she'll run. Vice-President Joe Biden, another potential candidate, plans to attend only the White House medal ceremony.