The senior Republican expressed concerns over what he said was a negative campaign by the Republican party under John McCain, the party's presidential candidate, and Sarah Palin, his running mate.
"I have said to Mr McCain that I admire all he has done. I have some concerns about the direction the party has taken in recent years. It has moved more to the right than I would like to see it," he said.
Powell also said that he was troubled by members of the Republican party using suggestions that Obama is a Muslim to attack him, pointing out that the Illinois senator is in fact a Christian and asking why it should be an issue anyway.
"I have heard senior members of my own party drop the suggestion that he [Obama] is a Muslim and he might be associated with terrorists. This is not the way we should be doing it in America," he said.
"Those images going out on Al Jazeera are killing us around the world. And we have got to say to the world, it doesn't make any difference who you are or what you are, if you're an American, you're an American."
Al Jazeera's Cath Turner, reporting from Washington DC, said that that there had been a demonisation of Muslims in the US since the September 11, 2001, attacks and that McCain supporters had brought up the issue at rallies.
There was much anger and distaste at McCain rallies for the idea that Obama is a Muslim, despite it being untrue, our correspondent said.
Powell said Palin, who would take over as president were McCain to fall ill or die during his presidency, was not ready for the role.
"She is a very distinguished woman and she is to be admired," Powell said. "But ... I don't believe she is ready to be president of the United States."
He also said McCain had also gone "too far" with negative advertising efforts over Obama's ties to William Ayers, a former 1960s radical, but that it was the economic crisis that had made up his mind to back the Illinois senator.
'Electrify the world'
Powell said that "all Americans should be proud, not just African-Americans," if Obama were to become the first mixed-race president after the elections on November 4.
"It would not just electrify our country, it would electrify the world," he said.
Powell appeared before the United Nations with what he said was evidence Saddam Hussein had weapons of mass destruction, helping to build the case for the US invasion of Iraq – an invasion Obama maintains was wrong.
Powell later called his UN speech a "blot" on his record.
He became a private citizen when George Bush won his second term as president and has largely refrained from entering public political discussions.
But on Sunday, Powell was clear in his desire to see a break from the last eight years of Republican policies.
"We need a president that will not just continue - even with a new face and some changes and some maverick aspects - who will not just continue basically the policies that we have been following in recent years."
Larry Wilkerson, Powell's former chief of staff, told Al Jazeera that it was "very painful" for him to have to break from McCain, whom he has know for a long time.
He said Powell "put his country ahead of his friendship, he put his country ahead of his party".
Asked if Powell's endorsement could be seen as based on racial solidarity, Wilkerson, who is white, Republican and has been campaigning for Obama, said he did not think so, explaining that Powell's point was that a candidate's race or religion should not matter.
He also said Powell's decision was not made because of anger with the Bush administration.
"I think it was made on ... the best person to lead America and ... to help lead the world," Wilkerson said.
Powell said that he did not intend to join the Democratic campaign as the race for the White House enters its final weeks and was not looking for a job in any Obama administration.
However, he refused to rule out serving if he were offered a post.
"I've always said if a president asks you to do something, you have to consider it," he said.
Obama said he was beyond honoured and deeply humbled to have Powell's support.
|McCain is trailing Obama in nationwide
opinion polls [AFP]
"And he knows, as we do, that this is a moment where we all need to come together as one nation – young and old, rich and poor, black and white, Republican and Democrat," Obama said.
Robert Gibbs, a spokesman for Obama's campaign, said the candidate "looked forward to taking advantage of his [Powell's] advice in the next two weeks, and hopefully over the next four years".
McCain, appearing on Fox News Sunday, was quick to play down the endorsement.
"Well, I've always respected and admired General Powell; we're longtime friends. This doesn't come as a surprise," he said, noting that he had received the support of other former secretaries of state, including Henry Kissinger, James Baker and Lawrence Eagleburger.
The Republican candidate is trailing his Democratic rival in nationwide opinion polls, as well as those in most key swing states that are likely to decide the election.
And McCain's cause was not helped by news on Sunday that Obama's campaign had raised more than $150m in September, giving it a huge spending advantage over the Republican candidate.
David Plouffe, Obama's campaign manager, said in an email to supporters that the campaign had added 632,000 new donors taking them to a total of 3.1 million contributors with an average donation of $86.
Obama's huge resources were possible because he opted out of the public financing system for the campaign. McCain chose to participate in the system, which limits him to $84m for the September-October stretch before the election.
McCain said that the amount raised meant there was a potential for fundraising abuses and went against the idea of keeping costs and spending under control.
"History shows us where unlimited amounts of money are in political campaigns, it leads to scandal," he said on Fox News Sunday.