Students burn pictures of U.S. President Barack Obama with former American presidents (L) and Obama shaking hands with former Israeli prime minister Ehud Olmert, in front of the former U.S. embassy during a gathering to mark the 29th anniversary of freezing Iran and US diplomatic ties in Tehran April 9, 2009. The United States cut off diplomatic ties with Tehran during the 1979-1981 hostage crisis in which a group of militant Iranian students held 52 U.S. diplomats hostage at the American Embassy for 444 days.