* Biden aggressively defends administration's record
* Ryan accuses Obama of weakness in leadership
* Democrats hope Biden can blunt Republican momentum
By John Whitesides and Andy Sullivan
DANVILLE, Ky., Oct 11 (Reuters) - U.S. Vice President Joe Biden and rival Republican Paul Ryan battled fiercely over foreign policy and the economy in a lively debate on Thursday, with Biden aggressively defending the administration's policies and dismissing Ryan's criticism.
Seeking to win back the momentum for the Democrats' campaign after a poor debate performance by President Barack Obama last week, Biden frequently went on the attack.
"With all due respect, that is a bunch of malarkey," Biden said when Ryan accused the White House of projecting an image of American weakness to the world.
But Wisconsin congressman Ryan, almost 30 years younger than his vastly experienced opponent, held his own in a series of testy exchanges.
The Republican joked that the gaffe-prone Biden should understand that "sometimes the words don't always come out the right way."
Biden took the offensive early, providing the emotion and passion that Obama was criticized for lacking in last week's debate with Republican challenger Mitt Romney.
Ryan stood his ground, and the vice presidential candidates for the Nov. 6 election frequently interrupted each other and talked at the same time.
In body language that might not go down well with undecided voters, Biden smiled and laughed sarcastically at points during the 90-minute debate in an apparent effort to dismiss Ryan comments.
Ryan said Americans were seeing the "unraveling" of Obama's foreign policy, and Biden attacked Romney as "not presidential" for holding a news conference on Libya just after a U.S. diplomatic compound was attacked and the ambassador killed.
Democrats were counting on a forceful performance from Biden to reclaim the momentum in the race for the White House after Obama's poor showing led to Romney taking the lead in polls with less than four weeks before the Nov. 6 election.
"Mr. Vice President, I know you're under a lot of duress to make up for lost ground, but I think people would be better served if we don't keep interrupting each other," Ryan said at one point.
"Well, don't take all the four minutes then," Biden responded. He later added: "I don't know what world this guy's living in."
Ryan said the Obama administration had given confusing information about the killing last month of the U.S. ambassador to Libya, Christopher Stevens.
"It took the president two weeks to acknowledge that this was a terrorist attack," the Wisconsin congressman said.
Biden vowed the administration would find the perpetrators of the attack and rectify mistakes in security at the diplomatic mission in Benghazi.
The two candidates sat across from each other at a table but the proximity did not lessen the conflict, as they aggressively went after each other.
Ryan defended Romney's secretly recorded video condemning the "47 percent" of the electorate that he said was dependent on government and considered themselves victims, calling it a mistake.
The 42-year-old Ryan, a seven-term congressman and chairman of the House of Representatives Budget Committee, is popular with conservatives for a budget plan that would slash government spending and creating a "voucher" system for the popular Medicare healthcare program for seniors. Democrats say it would leave some retirees paying more of their medical costs.
"We will not be part of any voucher plan, or the privatization of Social Security," Biden said.
Biden, 69, the former chairman of the Senate Foreign Relations and Judiciary committees, clearly was ready to point out the lack of international experience on the Romney-Ryan ticket.
Obama set an aggressive tone before the Biden-Ryan debate, accusing Romney of shifting toward the political center despite touting conservative credentials during the long Republican nomination contest.
But he also has a reputation for gaffes, including a recent remark that the middle class has been "buried for the last four years" - almost the span of Obama's presidency - by a bad economy.