KABUL, Afghanistan — The opening of a national constitutional assembly has been postponed again amid threats of violence from Islamic groups and concerns that the meeting could produce a paralyzing split between conservatives and reformists and damage chances for successful presidential elections next year.

Officials said Friday night that the assembly, which was set to begin today, has been delayed until Sunday only because some of the 500 delegates have had difficulty reaching Kabul from remote provinces. But security has been extremely tight as participants gather for the meeting, known as a loya jirga. Hundreds of Afghan and foreign troops are guarding the site at Kabul’s polytechnic university campus.

The loya jirga is being convened to debate and ratify a new constitution — the first since 1964, when Afghanistan was ruled by a monarch and had not yet experienced Soviet occupation, civil war and repressive Islamic rule. The assembly is a critical step in the country’s U.N.-mandated transition to democratic rule, and will almost certainly feature fiery confrontations among delegates from widely diverse backgrounds, including urban women’s activists, conservative tribal elders and former Islamic militia commanders.

One concern among outside observers is that the assembly may be hijacked by Islamic fundamentalists. More than two-thirds of the 344 elected male delegates are said to be associated with factions that want the new constitution to enshrine strict Islamic law and provide for a prime minister who can act as a counterweight to a powerful president.

Moreover, it is not yet clear who will chair the assembly or how that person — who can set and control much of its agenda — will be chosen. President Hamid Karzai reportedly wants to name a moderate former Afghan president, but religious conservatives are pushing for a direct election by the delegates, which could put someone from their ranks — probably Islamic factional leader Abdul Rassool Sayyaf — in the powerful post.