MOGADISHU - A special assembly tasked with voting on a new constitution for war-torn Somalia convened for the first time Wednesday, as the corruption-riddled government approaches the end of its mandate next month.

Billed as the key to lifting anarchic Somalia out of two decades of civil war, the end of the transitional government comes as regional forces have wrested a series of key strongholds from Al-Qaeda linked Shebab insurgents.

The 825-member National Constituent Assembly (NCA) -- chosen by traditional elders in a UN-backed process -- are due to meet for nine days to debate on a provisional constitution, before final ratification by a national referendum.

"It is encouraging to see that progress is being made," the UN Special Representative for Somalia Augustine Mahiga said in a statement ahead of the opening of the assembly.

"It marks significant progress towards ending the transition and providing the new political institutions for a stable and functional state in Somalia, after 21 years of political and civil strife," he added.

Hundreds of delegates crowded into the meeting hall -- a former police academy -- with heightened security provided by African Union troops and government forces, a reporter said.

The complicated process is seen as a key step as the Western-backed Transitional Federal Government (TFG) ends its mandate next month, after eight years of infighting and minimal political progress.

Somalia has been without a stable central government since the ouster of former president Siad Barre in 1991.

The draft constitution envisions a federal republic with laws "compliant with the general principles of Sharia" or Islamic law, and proposes a multi-party system with women "included in all national institutions".

All citizens, regardless of "sex, religion, social or economic status... shall have equal rights and duties before the law," the draft adds.

Anyone with a "record of serious crime or crimes against humanity" is officially barred from the assembly, which aims to include representatives from across society.

Officially, at least 30% are women, with members including the youth, religious scholars, traditional elders, the business community, academics and the diaspora.

The self-declared independent nation of Somaliland has refused to participate, although the current draft constitution lays claim to the north-western region as part of Somalia.

Bowed down by repeated droughts and riven by over two decades of conflict, Somalia is torn between rival clans, Islamist insurgents and the government, propped up by a 17,000-strong AU force.

Transitional institutions, including the presidency and the parliament, were set up in 2004 but must be replaced by permanent institutions by August 20.

A leaked UN report earlier this month accused the current government of "pervasive corruption" estimating as much as 70% of state revenues had been stolen or squandered.

However, President Sharif Sheikh Ahmed, fingered by the report, has dismissed the accusations and said he is "very confident" he will retain the country's top job.

In 2011, almost a quarter of total government expenditure -- over $12 million -- was "absorbed" by the offices of the president, prime minister and speaker, almost as much as was spent on security, the report said.