Sheik Aweys emerged from hiding to attend the talks
Somali Islamists and opposition leaders meeting in Eritrea have joined forces in a new alliance to overthrow Somalia's transitional government.
More than 300 delegates, including Islamist leader Sheikh Hassan Dahir Aweys, have approved a constitution and central committee.
A spokesman said the new movement will be called The Alliance for the Liberation of Somalia.
It aims to remove the Ethiopian-backed government by negotiation - or war.
"We have two-track options - first is the liberation of Somalia through military struggle, the second is through diplomatic efforts," said Zakariya Mahamud Abdi, spokesman for the Somali Congress.
The Alliance for the Liberation of Somalia (ALS) will have a 191-member central committee that will function as a parliament with a 10-person executive committee to be elected shortly.
The spokesman had a stark warning for Ethiopian troops, heavily deployed in Somalia since they rescued embattled transitional government forces last year.
"We warn Ethiopia to withdraw immediately. It is now or never and in a few weeks they will not have a route to withdraw," Abdi said.
Reporters at the Somali Congress for Liberation and Reconstitution in Asmara say the alliance is unlikely to be Islamist-led as the opposition is hoping to draw on the broad political support and fundraising opportunities of the Somali diaspora.
The participants want to see the Ethiopians out within two months
But observers say it will be interesting to see if a position is offered to the Islamist leader Sheikh Aweys, an architect of the Mogadishu insurgency, who has been in hiding since the Islamic Courts' Union was routed by the Ethiopian army last year.
In an interview with the Eritrean media, Sheikh Aweys, has dismissed US allegations that he is a "terrorist".
"I am a Somali nationalist fighting for a free and united Somalia," he said "and this is considered by the US administration to be terrorism."
Since their defeat by Ethiopia's vastly superior military force in December last year, the Islamists have resorted to guerrilla tactics, launching daily hit-and-run attacks on targets, mainly in Mogadishu.
The UN refugee agency says some 400,000 people have fled the fighting in the capital in the past four months as a result of the surge in violence.
The Islamists, along with other opposition leaders like Hussain Aideed, boycotted a reconciliation meeting sponsored by the transitional government last month.
Instead they chose to organise a meeting hosted by Ethiopia's arch-enemy, Eritrea, with the declared aim of "liberating Somalia from Ethiopia".
Ethiopian Prime Minister Meles Zenawi has said his troops will withdraw once African Union peacekeepers arrive in Mogadishu.
The AU force has failed to deploy effectively in Somalia so far
But pledges by AU nations to contribute troops to the planned 7,000-strong peacekeeping mission have yet to be honoured and so far only 1,600 Ugandan soldiers have been deployed.
Just days ago, a senior US official said the presence of Sheikh Aweys in Asmara was further evidence Eritrea gave sanctuary to terrorists.
The gathering of further intelligence could lead to Eritrea being named as a state sponsor of terrorism - followed by sanctions, the official warned.