George W Bush has been sworn in for a second term as US president.
Bush vowed to help end tyranny in the world
President Bush took the oath of office outside the Capitol building using a family Bible just before 1700 GMT.
"The survival of liberty in our land increasingly depends on the success of liberty in other lands," he said in his acceptance speech.
An estimated 500,000 people have braved the snow and the cold for the $40m (£21m) ceremony and parade - some to protest against Mr Bush's policies.
One protester managed to get within about 150m of the stand, shouting: "Hey Bush, what about the poor? How many have you sent to Iraq?"
Vice-President Dick Cheney was sworn in first, in accordance with the US constitution.
"The best hope for peace in our world is the expansion of freedom in all the world," Mr Bush told the gathering.
It is the policy of the US, he said, to support forces of democracy "with the ultimate goal of ending tyranny in our world".
"There is no justice without freedom, and there can be no human rights without human liberty," Mr Bush said.
"We do not accept the existence of permanent tyranny," he said.
"All who live in tyranny and hopelessness can know: the United States will not ignore your oppression, or excuse your oppressors. When you stand for your liberty, we will stand with you."
He said US allies should know that Washington honoured their friendship and respected their advice.
The BBC's Rob Watson in Washington says that, unlike four years ago, the president begins his second term after a comfortable election victory, but he still faces many challenges.
The war in Iraq is increasingly unpopular and opinion polls suggest Mr Bush's overall approval ratings are lower than any other re-elected president in recent years.
That rating currently stands around the 50% mark - the lowest for a returning president since Dwight Eisenhower in 1957.
But in a veiled reference to the splits over Iraq, Mr Bush said: "We have known divisions, which must be healed to move forward in great purposes - and I will strive in good faith to heal them."
Security measures for the inauguration were intense.
The entire area around Capitol Hill and the White House has been cordoned off, and more than 100 streets are off-limits to traffic, with the inauguration designated a national special-security event.
SCHEDULE OF EVENTS
1630 GMT: Ceremony begins
1700 GMT: Bush takes Oath of Office, delivers speech
1740 GMT: Ceremony ends
1930 GMT: Parade begins
2130 GMT: Parade ends
About 6,000 police and 7,000 military personnel are on duty for the inauguration, alongside an undisclosed number of secret service personnel.
Snipers have taken up positions on rooftops, bomb-sniffing dogs are on patrol and Patriot anti-missile batteries have been readied.
Manhole covers on Pennsylvania Avenue - the route of the parade - have been welded shut as a precaution.
"This is unprecedented when it comes to the level of security that will be in effect for the inauguration and those events that are surrounding it," Secret Service chief Ralph Basham said.
A number of "counter-inaugural" events have been planned, including an anti-war march through Malcolm X Park.
Protesters have planned to turn their backs on Mr Bush along the parade route.