Tens of thousands of Hamas supporters massed in Gaza City on Sunday to mark the 21st anniversary of the creation of the Islamist movement which violently seized control of the Gaza Strip last year. Busloads of demonstrators flooded into the city from across the densely populated territory, waving the green flags of the Islamic Resistance Movement. “Hamas has gone from stonethrowing to guns and rockets, from a support base of a few thousand people to a backing of millions in Arab countries and around the world,” one of the movement's top officials in Gaza, Mahmud Zahar, boasted on the Hamas website. “It has succeeded in striking at Israel's national security.” Hamas won an upset victory in Palestinian parliamentary elections in January 2006 but remains blacklisted as a terrorist organization by both the European Union and the United States as well as Israel. Hamas television put the turnout for the afternoon demonstration in Katiba Square in the city center in the hundreds of thousands. The head of the Hamas administration in Gaza, Ismail Haniyeh, was due to address the rally, which was intended as a show of strength in the Islamists' standoff with the West Bank-based administration of president Mahmoud Abbas. Haniyeh was expected to set out the Hamas position on a troubled six-month-old truce with Israel which runs out on Thursday after weeks of persistent violence. The Islamists accuse Israel of failing to honor its side of the bargain by easing its crippling blockade of aid-dependent Gaza. Israel accuses Hamas of failing to stop militant groups from raining rockets and mortar rounds on its southern towns. “There is no sense in extending the truce while the enemy is not respecting it and is keeping Gaza in a state of siege,” said Hamas spokesman Fawzi Barhum, without saying whether the Islamists intended to declare it over. Senior Israeli defense ministry official Amos Gilad, who led the Israeli side in the Egyptian-brokered negotiations for the original truce, was in Cairo on Sunday for talks on extending it. Despite a spate of tit-for-tat attacks since November 4 that has prompted some Israeli ministers to call for a major ground offensive against Gaza, Gilad has repeatedly spoken out in favor of extending the truce, arguing that there is no military solution. “Experience shows that military operations don't always solve problems in the Middle East,” he said late last month. “You have to find the optimal solution. To date no appropriate military solution has been found for the Strip.” Israel said on Sunday it was keeping border crossings with the Gaza Strip sealed off after rocket fire from the impoverished Palestinian territory. “This decision was taken after rocket and mortar fire on Saturday towards southern Israel,” said Peter Lerner, spokesman for the coordinator of Israeli activities in the Palestinian territories. The rockets fired from Gaza, which has been ruled by the Islamist movement Hamas since June last year, caused no victims or damage, Israeli police said. A Gaza ceasefire in force since June has been rattled over the past few weeks by by a string of tit-for-tat attacks between the Israeli army and militants, who fired dozens of rockets against southern Israel. An Israeli official said on Saturday that the government was willing to renew the truce with Hamas in and around the Gaza Strip if militants halt all attacks against the Jewish state. Top defense ministry official Amos Gilad is due in Cairo on Sunday for talks with Egyptian intelligence chief Omar Suleiman and senior officials to discuss the renewal of the Egyptian-brokered deal, which is set to expire on Dec. 19. Israel also says it will shortly release 229 Palestinian prisoners as a goodwill gesture to the western-backed government of President Mahmoud Abbas. The prisoners were to have been let out last week for the Muslim Eid Al-Adha holiday but Israel says Palestinian officials asked for their release to be postponed until Abbas returned from a pilgrimage to Mecca and was able to personally greet the freed prisoners.