The glad tidings for Obama came around 11.15 pm when major networks gave him 290 electoral votes to take him over the threshold of 270 required to win. Romney was reported as getting 203 electoral votes.

Four years ago, Obama was elected as the US' first African American President and he has now become the third consecutive President to retain office after George Bush and Bill Clinton.

As celebrations broke out across the country, a victorious Obama tweeted: "Four more years."

"We're all in this together. That's how we campaigned, and that's who we are. Thank you," said another tweet from Obama, the 51-year-old son of a Kenyan father and a White American mother.

Even pro-Republican Fox News conceded Obama's win as he bagged the first big contested prize of the night in Pennsylvania followed by New Hampshire, where Romney has a home and began his campaign, and Iowa.

In early results, Obama won his home State of Illinois as well as Romney's home State of Massachusetts, where Romney was Governor from 2003 to 2007.

Networks have also projected that Obama will win Wisconsin, the home of Romney's running mate Paul Ryan.

Democrats also held on to their majority in the Senate, picking up two seats in early returns with the possibility of a third while Republicans are expected to maintain majority in the US House of Representatives, which they currently control by 242-193.

List of US presidents

1.George Washington, 1789-1797

2.John Adams, 1797-1801

3.Thomas Jefferson, 1801-1809

4.James Madison, 1809-1817

5.James Monroe, 1817-1825

6.John Quincy Adams, 1825-1829

7.Andrew Jackson, 1829-1837

8.Martin Van Buren, 1837-1841

9.William Henry Harrison, 1841

10.John Tyler, 1841-1845

11.James Knox Polk, 1845-1849

12.Zachary Taylor, 1849-1850

13.Millard Fillmore, 1850-1853

14.Franklin Pierce, 1853-1857

15.James Buchanan, 1857-1861

16.Abraham Lincoln, 1861-1865

17.Andrew Johnson, 1865-1869

18.Ulysses S. Grant, 1869-1877

19.Rutherford Birchard Hayes, 1877-1881

20.James Abram Garfield, 1881

21.Chester Alan Arthur, 1881-1885

22.Grover Cleveland, 1885-1889

23.Benjamin Harrison, 1889-1893

24.Grover Cleveland, 1893-1897

25.William McKinley, 1897-1901

26.Theodore Roosevelt, 1901-1909

27.William Howard Taft, 1909-1913

28.Woodrow Wilson, 1913-1921

29.Warren Gamaliel Harding, 1921-1923

30.Calvin Coolidge, 1923-1929

31.Herbert Clark Hoover, 1929-1933

32.Franklin Delano Roosevelt, 1933-1945

33.Harry S. Truman, 1945-1953

34.Dwight David Eisenhower, 1953-1961

35.John Fitzgerald Kennedy, 1961-1963

36.Lyndon Baines Johnson, 1963-1969

37.Richard Milhous Nixon, 1969-1974

38.Gerald Rudolph Ford, 1974-1977

39.James Earl Carter, Jr., 1977-1981

40.Ronald Wilson Reagan, 1981-1989

41.George Herbert Walker Bush, 1989-1993

42.William Jefferson Clinton, 1993-2001

43.George Walker Bush, 2001-2009

44.Barack Hussein Obama, 2009-

Obama won a second term after defeating his Republican challenger Mitt Romney.