Inauguration: Barack Obama tells US to seize the moment

Barack Obama has told the American people to "seize the moment", in a speech in Washington DC inaugurating his second term as US president.

He issued a plea for political unity while embracing liberal causes such as immigration reform, gay rights and the fight against climate change.

Mr Obama, 51, who is the 44th US president, was sworn in for his second term by Chief Justice John Roberts.

Hundreds of thousands of people crammed the ceremony on the National Mall.

Former Presidents Bill Clinton and Jimmy Carter, along with dozens of senators, congressional leaders and other dignitaries, attended the event at the US Capitol.

Officials estimate about 700,000 people were to attend the inauguration, down significantly from 2009, when about 1.8 million people witnessed Mr Obama be sworn in as America's first black president. About 260,000 people attended George W Bush's second inauguration ceremony in 2005.

In the evening, the Obamas donned formal attire for two inaugural balls in Washington, where 40,000 people were expected to attend.