Edward Kennedy recently underwent surgery for brain cancer
US Democrats have gathered in Denver, Colorado, to nominate Barack Obama as the first African-American candidate for a presidential election.
House Speaker Nancy Pelosi said they met at a defining moment in history and praised Mr Obama's vision and values.
Senator Edward Kennedy, recently diagnosed with brain cancer, spoke of the "new hope" Mr Obama offered the US.
The Democrats hope the event will show the Illinois senator's personal side and heal the rifts of the primary race.
Mr Kennedy, the 76-year-old scion of the iconic Democratic family, appeared on stage to loud cheers from the party faithful.
"I have come here to stand with you to change America, to restore its future, to rise to our best ideals and elect Barack Obama president of the United States," he said.
His niece, Caroline Kennedy, who had introduced a video to Mr Kennedy just beforehand, paid her own emotional tribute to both him and Mr Obama.
"Their stories are very different but they share a commitment to the timeless American ideals of justice and fairness, service and sacrifice, faith and family," she said.
"Leaders like them come along rarely. But once or twice in a lifetime, they come along just when we need them most."
Michelle Obama is due to make a keynote speech in which she will praise her husband as someone who would make an "extraordinary" president and who shares the values dear to many Americans.
Mr Obama's half-sister, Maya Soetero-Ng, used her speech to describe a shared upbringing in which she and her brother learned that with hard work and imagination they could "dream the improbable".
Mrs Pelosi spoke of Mr Obama's "bold" vision for the nation's future and described him as "honouring American values, a belief in personal responsibility, in community, in hard work".
She also paid tribute to the campaign of Mr Obama's chief rival for the Democratic nomination, Senator Hillary Clinton, saying: "Our party and our country has been strengthened by her candidacy."
Opening the four-day convention, Democratic Party chairman Howard Dean also sought to bridge any divisions within party ranks.
He said: "Looking out from this podium tonight, I see this diverse assembly of Democrats as a testament to the strength and unity of our party."
Mr Obama, 47, will formally accept the party's nomination when he addresses a crowd of 80,000 at a sports stadium on Thursday night, fresh from touring electoral battlegrounds.
Speaking to reporters in Illinois on Monday, Mr Obama said his speech would be "workman-like".
"I'm not aiming for a lot of high rhetoric," he said. "I'm much more concerned with communicating how I intend to help middle-class families live their lives."
Party officials have downplayed any scope for discord after Mr Obama chose veteran Senator Joseph Biden as his running mate.
But analysts have predicted that some of Mrs Clinton's backers, angry that she did not win the nomination and was never seriously considered as a vice-president choice, could try to upset the party.
Monday: Michelle Obama speech on Obama the man; tribute to Ted Kennedy
Tuesday: Hillary Clinton speech; keynote speech by former Virginia governor Mark Warner
Wednesday: Speeches by Bill Clinton and Joe Biden; vote to confirm Barack Obama as party's candidate
Barack Obama to accept nomination with speech in stadium
Mrs Clinton herself, addressing delegates from her home state of New York, urged them to throw their support behind Mr Obama.
"We are united behind Barack Obama and Joe Biden and we're going to make sure that we win on November 4, so let's have a great election," she said.
The official party line is that all is sweetness and light in Denver, BBC North America editor Justin Webb reports from the city.
However, our correspondent adds, there may still be a potential flash-point on Wednesday night.
A roll-call of delegates will be taken and Mrs Clinton's supporters will get the chance to make a symbolic effort to have her nominated.
Although she has asked them to switch their formal allegiance to Mr Obama, some Clinton supporters may not play ball, our correspondent notes.
A Wall Street Journal/NBC poll published last Wednesday suggested that only 52% of people who voted for Hillary Clinton during the primaries had so far decided to vote for Mr Obama in November.
About one-fifth said they would vote for the Republicans' John McCain while 27% had not yet decided how they would vote, according to the survey.
More than 4,000 Democratic delegates and tens of thousands of officials, activists, protesters and journalists have descended on Denver for the four-day convention.
Monday's focus is on the White House hopeful's life story. After being attacked by Mr McCain's campaign as a "celebrity", Mr Obama has been trying to portray himself as an ordinary family man.
Meanwhile, the McCain campaign has been raking through the Democratic primary archives for quotes by Hillary Clinton and others for use in adverts and press releases questioning Mr Obama's experience.
"The power to this message is that you're using the Democrats' own words," said Republican strategist Scott Reed.
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