Barack Obama has narrowly won the key battleground state of Florida, according to final results released four days after the election.

Results released Saturday show the president carried the state with 50 per cent of the vote, about 74,000 votes ahead of Romney's 49.1 per cent. That ensures that Florida's 29 electoral college votes will all go to Obama.

Florida's final vote announcement had been held up by a late flood of absentee ballots in Miami-Dade County, record turnout, and long lines that saw voters in many parts of the state unable to actually cast their votes until early Wednesday morning.

The win gives Obama 332 electoral college votes – comfortably over the 270 needed to clinch victory. Mitt Romney won 206.

Obama's win was assured shortly after 11 p.m. ET Tuesday night when networks projected that, even without Florida, the president had won enough states to push him over the top.

The president carried Florida in part because heavy support from black, Hispanic and younger voters. Exit polls conducted for The Associated Press showed Obama was favoured by more than 9 of 10 black voters and 3 of 5 Hispanic voters in Florida. The president also was the choice of two-thirds of voters under age 30.

With files from The Associated Press