A sea of people for Obama inauguration

WASHINGTON (Reuters) - Barack Obama prepared to make history on Tuesday as the first black U.S. president, riding a wave of public optimism he will need to tap to deal with the worst economic crisis in 70 years and two wars.

Hundreds of thousands of people, bundled up against the cold and in a festive mood, packed Washington's Mall, which stretches 2 miles (3 km) from the Capitol to the Lincoln Memorial on the Potomac River, and along Pennsylvania Avenue to the White House.

Obama, a Democrat, was due to be sworn in as the 44th U.S. president just before noon EST (5 p.m. British time), taking over from Republican George W. Bush.

The inauguration was taking place amid unprecedented security. About 8,000 police were deployed and a total of 32,000 military personnel were on duty or on standby.

The U.S. Homeland Department said they were investigating a potential threat to the inauguration.

"This information is of limited specificity and uncertain credibility," Homeland Security spokesman Russ Knocke said, adding that Obama's aides had been briefed on the issue.

This will be the first inauguration of a new president since the September 11, 2001, attacks. Obama's election has stirred white supremacists to anger, sparked arrests during his campaign and raised assassination fears.

U.S. authorities arrested a U.S. man last week on suspicion of threatening to kill Obama based on statements he posted on a website.

(Writing by Ross Colvin; Additional reporting by Jeff Mason, Jim Wolf and Randall Mikkelsen)