This is the astonishing moment a book was apparently hurled at the head of President Barack Obama during a campaign rally in Philadelphia.

The flying missile narrowly missed hitting the President yesterday.

It is not clear what the book was, where it came from in the crowd, or why it was thrown at Mr Obama - who did not appear to notice the danger.

Duck, Mr President! A book narrowly misses Barack Obama's head after it is thrown on stage during a rally in Philadelphia today

Close call: It is not clear where the book came from in the crowd - or what type of book it was

But it is expected that there will be fallout from the security breach as the Secret Service investigates how close the President came to danger.

The rally was clearly an eventful one - other images showed a naked man being led away in handcuffs by police.

It is not clear if the man was involved in the book-throwing incident - or why he was not wearing any clothes.

The bizarre incident recalled the moment in 2008 when an angry Iraqi journalist hurled a shoe at then-U.S. President George Bush during a press conference in Baghdad.

The surprisingly nimble Mr Bush ducked the shoe - and the moment became immortalised with online parodies and internet video games.

But the incident was also marked with controversy as U.S. media questioned why the Secret Service - whose members are supposed to be willing to take a bullet for the President - were not close enough to Mr Bush to deflect the attack.

It is expected that the same questions will be asked about yesterday's incident.

It came as Mr Obama tried to rally voters in Phildelphia to use to three weeks left before congressional elections to go to the polls.

The president relied on an oft-used speech as he addressed the crowd in the city's Germantown community with the driving cadences that swept him into the White House two years ago.

Er - what? A woman recoils as a naked man is escorted from Mr Obama's rally in Philadelphia today. It is not clear if the man was involved in the book-throwing incident - or why he was not wearing clothes

Campaign commentary: A police officer tries to protect the naked man's modesty with a strategically placed 'Vote 2010' sign

 

He and the Democratic party know, however, that this year finds Democrats imperiled because of what has become known as the 'enthusiasm gap', with party voters expected to stay away from the polls.

That could be disastrous in a nationwide vote that was widely believed to hand the majority in the House of Representatives and, perhaps, the Senate to Republicans.

There are ample reasons for Republican optimism. Poll after poll shows deep voter discontent and even anger at Mr Obama's and congressional Democrats' leadership. Mr Obama and his fellow Democrats are being blamed for the slow economic recovery and continuing high unemployment.

On Friday, the Bureau of Labor Statistics - in the last major economic news before the November elections - delivered another painful blow to Democrats: The U.S. lost 95,000 jobs in September and unemployment remained stubbornly stuck at 9.6 per cent.

In another complication for Democrats, the government is expected to announce this week that more than 58 million Social Security recipients will go through another year without a cost-of-living increase in their monthly benefits.

It would mark only the second year without an increase since automatic adjustments for inflation were adopted in 1975. The first year was this year.

'I think the pundits are wrong. I think we're going to win. But you've got to prove them wrong,' Mr Obama said, jabbing his finger toward the audience.

'They're counting on you staying home. If that happens they win.'

Vice President Joe Biden joined Obama at the rally.

Rallying the voters: Mr Obama, shown here making his speech in Philadelphia today, intended to bridge the 'enthusiasm gap' and bring voters to the polls

Team: Mr Obama was joined by Vice President Joe Biden at the rally today

It was the second of four large rallies designed to recapture some of the big-stage excitement that Mr Obama created in 2008 with stirring speeches to thousands of young and first-time voters.

The President spoke to more than 17,000 people last week in Madison, Wisconsin, where many more thousands watched on screens in an overflow area.