U.S. President elect Barack Obama and his wife Michelle wave to their supporters after Obama gave his victory speech during an election night gathering in Grant Park on November 4, 2008 in Chicago, Illinois. Obama defeated Republican nominee Sen. John McCain (R-AZ) by a wide margin in the election to become the first African-American U.S. President elect. (Getty Images/AFP)

Chicago, Nov 4 (Reuters) - President-elect Barack Obama told ecstatic supporters on Tuesday that 'change has come to America' and called on all Americans to unite and meet pressing challenges.

Taking the stage in his home city of Chicago after defeating his Republican rival John McCain following a bitter election campaign, Obama told an enormous outdoor victory rally that the road ahead would be tough.

'Our climb will be steep,' Obama said to the crowd of more than 200,000 supporters who crammed into the rally at Grant Park in his home city of Chicago.

'We may not get there in one year or even one term, but America -- I have never been more hopeful than I am tonight that we will get there.'

Obama, whose campaign was marked by moments of soaring rhetoric, took stock of what it meant to be elected the first black U.S. president.

'It's the answer that led those who have been told for so long by so many to be cynical, and fearful, and doubtful of what we can achieve to put their hands on the arc of history and bend it once more toward the hope of a better day,' Obama said.

'It's been a long time coming, but tonight, because of what we did on this day, in this election, at this defining moment, change has come to America,' said the 47-year-old Illinois senator.

The atmosphere at the victory celebration was electric, as the crowd chanted Obama's election slogan'Yes, we can' and elated supporters and campaign staff hugged one another.

Obama paid tribute to his defeated rival as a patriot, hailing McCain's status as a former prisoner of war in Vietnam and saying he hoped to work with him.

'He has endured sacrifices for America that most of us cannot begin to imagine, and we are better off for the service rendered by this brave and selfless leader,' Obama said.

People in the crowd put their cameras up high to get a shot of Obama, or called friends and family on cell phones to share the moment.

'It is incredible that this is happening in my lifetime,' said Julie Mierwa.

Some, however, voiced concern that maybe Obama had promised too much.

'I don't know whether he has over-extended himself and if he will be able to live up to the expectations of him. But today is his day. It was incredible,' said Jass Hylton.

Outside the rally, there was bumper to bumper traffic and locals cranked up their radios to hear Obama's address.

Chicago officials put the crowd at the rally at more than 200,000.