With security tight following Taliban threats, Afghans prepared on Sunday for a crucial debate on a constitution designed to take their war-battered country to its first free elections. Hundreds of police, soldiers and foreign peacekeepers manned a tight security cordon in a 2-mile radius of the venue at a polytechnic campus in the city's west. The Loya Jirga was originally due to start last Wednesday and first postponed until Saturday amid protracted wrangling, and then to Sunday. The debate, expected to run from 10 days to several weeks, will be held in a giant tent donated by a German NGO that evokes the tradition of Afghan tribal meetings throughout the ages. Tanks and armored personnel carriers from the NATO-led multi-national peacekeeping force and the Kabul garrison guarded key intersections and ridges overlooking the site. Taliban fighters have threatened to disrupt the gathering and said anyone attending deserved to be killed. No chances have been taken, despite an absence of incidents in recent days. In distant southern provinces bordering Pakistan, where fighters are most active, thousands of U.S.-led forces mounted patrols and manned checkpoints in what the U.S. military calls its biggest drive yet against the Taliban and its allies. It says "Operation Avalanche" across an area the size of California has helped prevent attempts to disrupt the Loya Jirga. The outcome of the meeting and Afghanistan's progress toward elections is seen as a key test for rebuilding on America's other, even more troubled front, Iraq. **PHOTO CAPTION*** An Afghan boy carries bread to his home as a jet forms vapor trail behind, in Kabul, Afghanistan, Thursday, Dec. 11, 2003. (AP Photo/Aijaz Rahi)