Democratic Senator Barack Obama is projected to have won two key battleground states, giving him a strong lead in the presidential race.
His projected win in Ohio, won by the Republicans in 2004, is a crucial stepping stone to the White House.
He also took Pennsylvania. This was one of John McCain's main targets, and experts say it will be difficult for him to win without it.
Amid reports of "unprecedented turnout" other results have gone as expected.
Mr Obama is hoping to become the first black president of the United States.
The main developments include:
* Mr Obama is projected to have taken: Ohio, Vermont, New Hampshire, Pennsylvania, Illinois, Delaware, Massachusetts, District of Columbia, Maryland, Connecticut, Maine, New Jersey, Michigan, Minnesota, Wisconsin, New York, Rhode Island.
* Mr McCain is projected to have won: Kentucky, South Carolina, Oklahoma, Tennessee, Arkansas, Alabama, Kansas, North Dakota, Wyoming, Georgia, Louisiana, West Virginia.
* In addition, some US networks call New Mexico for Mr Obama - which would be a gain.
* The Democrats made early gains in the Senate race, seizing seats from the Republicans in Virginia, North Carolina and New Hampshire.
* Exit polls suggest the economy was the major deciding factor for six out of 10 voters.
* Nine out of 10 said the candidates' race was not important to their vote, the Associated Press reported. Almost as many said age did not matter.
BBC