Donald Trump sworn in as 45th US president

The 70-year-old Republican billionaire placed his left hand on a bible used by Abraham Lincoln and recited the 35-word oath spoken since George Washington, and then launched into a much-anticipated inaugural address.

"We, the citizens of America, are now joined in a great national effort to rebuild our country and restore its promise for all of our people," Trump told a crowd of hundreds of thousands massed on the National Mall.

"Together we will determine the course of America, and the world, for many, many years to come," he said.

"From this day forward, it's going to be only America first."

Trump placed the battle against Islamic extremism at the heart of his foreign policy, vowing to work with allies to destroy the jihadist threat.

"We will reinforce old alliances and form new ones and unite the civilized world against radical Islamic terrorism, which we will eradicate completely from the face of the earth," he declared.

Trump ended his address by vowing to "make America great again".

A crowd of hundreds of thousands looked on, including outgoing president Barack Obama and Trump's defeated election opponent Hillary Clinton - who narrowly missed out on becoming America's first female president.

Mike Pence was also sworn in as the vice president of the United States at the event.

Trump chose Pence, the former governor of Indiana, as his running mate last summer.