BOSTON, United States - US Republican presidential nominee Mitt Romney early Wednesday morning went up the stage at his Boston headquarters, conceding the presidential election he just lost.

"I have just called President Obama to congratulate him on his victory," said a smiling Romney, who fell short on his second bid to become president.

US Republican presidential nominee Mitt Romney gives his concession speech after losing the election to US President Barack Obama during his election night rally in Boston, Massachusetts, Nov 7, 2012. [Photo/Agencies]

Romney continued to say in this "time of great challenge for America, I pray the president will be successful in guiding our nation," and thanked his family, supporters and running mate Paul Ryan.

Romney urged Republicans and Democrats to work together to avoid party bickering, and said leaders should reach across the aisles to solve the nation's myriad of problems.

Obama, who handily beat Romney after sweeping nearly all of the critical battleground states, is to speak soon at his Chicago headquarters.

Republican presidential nominee Mitt Romney delivers his concession speech during his election night rally in Boston, Massachusetts, Nov 7, 2012. [Photo/Agencies]