Donald Trump has been sworn in as the 47th US president.
Tech billionaires, cabinet nominees and former presidents are all at the ceremony in the rotunda of the US Capitol.
Minutes before Trump took the oath, Joe Biden issued pardons for members of his family.
Trump declared a "national emergency" at the US-Mexico border, and scrap government diversity programmes as part of a blitz of executive orders in the coming hours.
He was also expected to declare drug cartels as terrorists.
But he won't announce tariffs on trade partners today, officials say.
Trump began his speech as president by addressing his fellow presidents and others attending the event, including former Vice-President Kamala Harris and former President Joe Biden.
"The golden age of America begins right now," he says. "From this day forward, our country will flourish and be respected.
"I will very simply put America first."
Trump continued his speech by laying out his vision for America.
"Our sovereignty will be reclaimed, our safety will be restored, the scales of justice will be rebalanced," he says, adding that "the vicious violent and unfair weaponisation" of the US Justice Department will end.
"Our top priority will be to create a nation that is proud, prosperous and free," he says.
President hits out at Biden-era policies
Trump used his first speech to attack the Biden administration and its handling of the migrant crisis.
He says the country's challenges will be "annihilated", adding that the US is confronting a crisis of "trust over radical and corrupt establishment".
The previous administration, he says, has provided sanctuary and protection for "dangerous criminals" who have illegally entered our country.
The government has given "unlimited funding to the defence of foreign borders" but refuses to defend American borders, he claims.
"We now have a government that cannot manage even a simple crisis at home," Trump says.
Trump goes on to say that America will soon be "greater, stronger, and far more exceptional than ever before".
He says he returns to the presidency "confident and optimistic" and that it is the "start of a thrilling new era of national success".
"Sunlight is pouring over the entire world and America has the chance to seize this opportunity like never before," he adds.
