Washington/Chicago  - Democratic candidate Barack Obama won a historic US presidential election Tuesday night, becoming the first African-American ever to lead the United States.

Obama, 47, had a commanding victory over Republican rival John McCain, 72, capturing a series of reliable Republican states including Virginia, Ohio and Florida that voted for President George W Bush in the last two elections.

McCain called Obama to concede victory. In a speech in Phoenix, Arizona, he recognized the "historic" nature of an African American taking the highest office in the land and promised to help Obama unite a country that remains sharply divided.

"In a contest as long and difficult as this campaign has been, his success alone commands my respect for his ability and perseverance," McCain told a crowd of downcast supporters, who booed at McCain's mention of Obama.

In Chicago, a deafening roar went out from the tens of thousands of supporters gathered in Grant Park as networks projected the Illinois senator's win. Supporters were hugging each other and shouting out "Yes we can! Yes we can!"

"I never thought I would see this in my lifetime," said Jessica Rose, a 26-year-old African American woman from Chicago. "It's all like a fairy tale, it really is."

The popular vote total stood at 51 per cent for Obama and 49 per cent for McCain, with two-thirds of precincts around the country reporting.

But it is state victories that were key in the country's state-by- state, winner-takes-all electoral system. A total of 270 electoral votes is needed. The count stood late Tuesday at 333 electoral votes for Obama and 155 for McCain.

Obama was put over the top as US networks projected victories in California, Washington and Oregon as soon as polls in those states closed at 11 pm (0400 GMT).

Obama had already taken a commanding lead in the US presidential election before the official projection, capturing the key swing states of Pennsylvania and Ohio earlier in the night and leaving McCain with an almost impossible uphill battle to keep his hopes alive.

No candidate has captured the White House without Ohio's 20 electoral votes since 1960 and no Republican has ever won the presidency without it. Similarly in Virginia, no Democrat has won the state since 1964.

Pennsylvania, with 21 electoral votes, had been considered McCain's best shot of capturing a reliable Democratic state. McCain did not win a single state taken by Democrat John Kerry four years ago.

Some of the other key states captured by Obama included Michigan, Wisconsin, New Hampshire, New Mexico and Iowa. Georgia, South Carolina, Louisiana and Texas were some of the largest taken by McCain.

Democrats had worked on the hope that eight years of President George W Bush's unpopular policies in Iraq and the slumping economy would convince voters to hand them control of the White House and strengthen their control of Congress.

The Democrats picked up at least four seats from Republicans in the Senate and were poised to make significant gains to widen their majorities in both congressional houses, network projections showed.

The faltering US economy was by far the top concern of voters heading to the polls in Tuesday's general election, according to initial exit polls reported by US broadcaster CNN.

A full 62 per cent of voters ranked the economy as the key issue of this election, compared to 10 per cent invoking the war in Iraq and 9 per cent rating terrorism or health care highest.

Millions already voted in recent weeks in early or absentee voting allowed in 31 states, including key battlegrounds Florida, North Carolina, Colorado and Nevada.

Millions of voters waited patiently in serpentine queues early Tuesday to cast ballots. Many had started lining up before dawn, and some braved pouring rain to cast their ballots.

In Chicago, the disproportionately young crowd awaiting Obama's rally had a large representation of both whites and blacks, with small numbers of Latinos and Asians.

It was a bittersweet end to the 21-month campaign for Obama, whose grandmother, Madelyn Dunham, 86, passed away overnight Sunday after a battle with cancer, the Illinois senator revealed Monday.