US President Donald Trump takes the oath of office with his wife Melania and son Barron at his side, during his inauguration at the US Capitol in Washington, US, January 20, 2017. [Photo/Agencies]

Donald Trump took power as the 45th president of the United States on Friday and pledged to pursue "America First" policies in an inaugural address that was a populist, anti-Washington rallying cry.

Sketching a bleak vision of a country he said was ravaged by rusted-out factories, crime, gangs and drugs, Trump indirectly blamed his predecessors in the White House for policies that helped the establishment at the expense of struggling families.

He said the inauguration represented not just the peaceful transfer of power from one party to another. "We are transferring power from Washington, DC, and giving it back to you the people," he said.

"For too long," he said, "a small group in our nation's capital has reaped the rewards of government while the people have borne the cost. Washington flourished, but the people did not share in its wealth. Politicians prospered, but the jobs left and the factories closed. The establishment protected itself but not the citizens of our country. Their victories have not been your victories. Their triumphs have not been your triumphs."

He vowed to reverse that trend and make America first. "The forgotten men and women of our country will be forgotten no longer."

"From this moment on, it's going to be America First," the Republican told hundreds of thousands of people gathered on the grounds of the National Mall after taking the oath of office on the West Front of the US Capitol.

"The time for empty talk is over," he added later. "Now arrives the hour for action. Do not allow anyone to tell you it cannot be done."

Four past US presidents, three Democrats and a Republican, sat nearby. Scattered street protests erupted against Trump elsewhere in Washington.

"Every decision on trade, on taxes, on immigration, on foreign affairs, will be made to benefit American workers and American families," Trump said, describing the country's social and economic ills as "American carnage."

Trump, 70, takes over a country divided after a savage election campaign. A wealthy New York businessman and former reality TV star, he will set the country on a new, uncertain path at home and abroad.

Trump's inaugural address revisited the themes of the campaign rally speeches that carried him to an improbable victory on Nov 8 over Democrat Hillary Clinton, who attended the ceremony with her husband, former President Bill Clinton.

Under pressure to unite the country after the bitterly fought campaign, Trump said that through allegiance to the United States, "we will rediscover our loyalty to each other" and called for a "new national pride" that would help heal divisions.

Abroad, Trump signaled the possibility of a more aggressive approach to Islamic State militants than his immediate predecessor, Democrat Barack Obama.

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

After repeating the 35-word oath of office, Trump stretched his arms wide and hugged his wife, Melania, and other members of his family. Ceremonial cannon blasts fired.

The transition from a Democratic president to a Republican took place before a crowd of former presidents, dignitaries and hundreds of thousands of people on the grounds of the National Mall. The crowd stretched westward on a cool day of occasional light rain.