In the largest public campaign to date, tens of thousands of Israelis have signed a petition calling on President Shimon Peres to leverage his diplomatic stature in order to secure the freedom of Jonathan Pollard, a former intelligence analyst jailed in a U.S. prison for the past 27 years on charges of spying for Israel.

Peres on Saturday night departed on a six-day trip to the United States, where U.S. President Barack Obama will award him later in the week the Presidential Medal of Freedom -- the most prestigious civilian honor bestowed by the U.S. administration.

The Israeli president is also scheduled to hold meetings with several top U.S. officials to discuss Iran, the peace process and other regional issues.

Peres issued a formal request for Pollard in early April, following a highly emotional meeting with the jailed agent's wife, Esther Pollard, but officials here contend that the Medal of Freedom, which will be presented to Peres in a gala award ceremony that will be attended by top former and current officials, is a rare, possibly the last opportunity to press Obama to release Pollard, whose deteriorating health has forced his evacuation for emergency medical treatment a number of times in recent years.

On Wednesday, shortly before the award ceremony, Peres will receive a petition, which has been signed by 70,000 Israelis in recent months.

"Out of deep fear for Jonathan's welfare and a desire to see his spending the rest of his days as a free man, we ask you, President Peres, to leverage your unprecedented diplomatic stature and close relations with President Obama for Jonathan's release," the petition reads.

Heading the list of signatories are prominent Israeli public figures who until today have not rallied in a joint call to release the jailed spy: internationally-acclaimed authors A.B. Yehoushua, Amos Oz and David Grossman; Nobel laureates Dan Shechtman, Israel Oman and Aaron Ciechanover; chief rabbi Yona Metzger; and some of the country's revered actors, singers and former politicians.

Also on the list is Gilad Shalit, a former Israeli POW who spent more than five years in Hamas captivity in the Gaza Strip and released in a landmark prisoner swap deal last October.

"I hope and pray that Peres will succeed in using all his powers as both president and a man to convince Obama to free Pollard -- on humanitarian grounds ... Pollard sinned 27 years ago, but served out his punishment. He should be freed before it is too late," Shalit said Saturday.

America-born Pollard, a former civilian intelligence analyst for the U.S. Naval Investigative Service (NIS), was sentenced to life in jail in 1986 for handing top-secret documents to Israeli intelligence, among them a manual that detailed America's global electronic surveillance network.

Israel officially denied any connection to Pollard following his capture in 1985, a policy it maintained until the late 1990s. Senior Israeli leaders, however, have since lobbied for his release, including Prime Minister Benjamin Netanyahu, who visited Pollard at the maximum-security Butner Federal Correctional Complex in Butler, North Carolina, in 2002.

A long list of high-profile U.S. officials, including senators, an ex-CIA director and the former chairman of the Senate Intelligence Committee, have also pleaded for Pollard's release in recent years, yet their calls have fallen on deaf ears.

Lawrence Korb, a former assistant secretary of defense in the Regan administration, published an op-ed in the Los Angeles Times in October 2010 in which he wrote that Pollard's punishment "doesn 't fit his crime."

"Pollard has already served far too long for the crime for which he was convicted, and by now, whatever facts he might know would have little effect on national security," Korb wrote.

Spokesmen for the "Free Pollard" lobby, which is headed by rightist Israeli lawmakers, estimated the chances that Obama would announce a clemency during the ceremony to honor Peres as "low," but expressed hope that the meeting between the two leaders will convince Obama to pardon Pollard towards the end of his first term in office.