U.S. President Barack Obama gave a rousing victory speech to a packed crowd in Chicago early Wednesday morning, thanking voters for electing him to another term and congratulating his Republican opponents for a hard-fought battle.

"We are an American family and we rise as one people," he told the crowd.

"I want to thank every American, who participated in this election - whether you voted for the very first time, or waited in line for a very long time. By the way, we have to fix that."

"We will disagree, sometimes fiercely, about how to get there. Progress will come in fits and starts."

"Whether I earned your vote or not, I have listened to you, I have learned from you, and you've made me a better president. I return to the White House more determined and more inspired than ever."

"Tonight, you voted for action, not politics as usual. You elected us to focus on your jobs, not ours. In the coming weeks and months, I am looking forward to working with leaders of both parties. "

Earlier, Republican candidate Mitt Romney conceded the U.S. presidential election to President Barack Obama after a bitterly fought battle.

"This is a time of great challenges for America and I pray that the president will be successful in guiding our nation," Romney told supporters after calling Obama to congratulate him.

In a short concession speech, Romney called for bipartisanship in Washington.

"I believe in America. I believe in the people of America," he told supporters in Boston. "I ran for office because I'm concerned about America. This election is over, but our principles endure."

"I so wish that I had been able to fulfil your hopes to lead your country in a new direction," Romney said before being joined on stage by his wife, Ann and their family, along with running mate Paul Ryan, "but the nation chose another leader, and so Ann and I join with you to earnestly pray for him and for this great nation."

Obama defeated Romney in a series of key swing states, despite the weak economic recovery and stubbornly high unemployment that dogged his campaign.

Television networks called the election late on Tuesday, but the Romney campaign waited more than an hour to agree on the results in Ohio.

U.S. broadcasters declared Barack Obama the winner over challenger Mitt Romney after forecasting that the president had won the decisive
state of Ohio.

Obama thanked his supporters late Tuesday in a Twitter message as he secured enough electoral votes to win a second term.  "We're all in this together. That's how we campaigned, and that's who we are. Thank you," he wrote on his Twitter feed.

Obama rode signature first-term accomplishments including the auto industry bailout and the killing of 9/11 terrorist attack mastermind Osama bin Laden to another four years in the White House.

His victory ensures continued implementation of signature health care and financial sector reform laws and could bring higher taxes for the wealthy as part of deficit-cutting efforts.