Afghan President Hamid Karzai faced an open rebellion Wednesday among powerful faction leaders opposing his drive for a strongly centralized presidency during a historic constitutional council taking place in the capital. Some 500 delegates to the grand council, or loya jirga, have spent 11 days debating a draft put forward by Karzai's government. It foresees a tolerant Islamic state under a strong presidency, and is supposed to pave the way for landmark elections next summer. The tension surrounding the gathering was underlined early Thursday when a bomb damaged a U.N. guest house on the other side of the Afghan capital. No one was injured. The attack was at least the third in the capital since the council began on Dec. 14. However, none has caused injury or serious damage. The U.S. military has warned that Taliban fighters and their allies want to disrupt the gathering. "The presidential form of government is known all over the world. The powers are known, the limitations are known," Karzai told reporters on the steps of his Kabul palace. "We should be making a constitution that reflects that system, not a confusion of it." But Burhanuddin Rabbani, a leader of the Northern Alliance faction who was president during Afghanistan's ruinous 1992-1996 civil war, said some delegates feared the charter would produce a dictatorship. "In Third World countries, presidents have passed power to their sons and the result has been bloodshed and coups," he said in the huge tent where the meeting is being held. "If the presidential system is accepted, the delegates will ask for a strong parliament." Afghan and U.S. officials say they are confident a majority of the council wants a presidential system, which Karzai argues is critical to law and order in a country where the ousted Taliban regime once provided haven to al-Qaida network. Karzai appears to have rallied fellow Pashtuns - the country's largest ethnic group and from which the Taliban drew their main support - but delegates from smaller groups are calling openly for power to be spread more widely. Another top Northern Alliance figure, Uzbek strongman Abdul Rashid Dostum, came out Monday in favor of a parliamentary system while pleading for minority languages to be given official status. Meanwhile, an explosion went off outside a United Nations guest house between the main peacekeeping base and presidential palace in Kabul early Thursday, causing slight damage but injuring no one, witnesses and officials said. The blast ripped through a wall outside the guest house just before dawn, but none of the local or foreign U.N. employees staying at the two-story building were hurt, they said. Some windows of nearby residential houses were also shattered by the explosive device, which had been hidden in a pressure cooker, said Kabul's deputy police chief, Khalil Aminzada. Residents and shopkeepers cleared shards of glass and debris caused by the blast as the International Security Assistance Force (ISAF) of peacekeepers investigated the incident not far from its headquarters. **PHOTO CAPTION*** Afghan President Hamid Karzai speaks at the Presidential Palace in Kabul, Dec. 24, 2003. As Afghanistan's constitutional assembly nears voting on the country's future, Karzai reiterated the need for a strong presidential system that has raised loud protests from opponents. (Photo by Kimimasa Mayama/Reuters )