More than 6,000 people in Mogadishu cheered and railed against mainly Christian Ethiopia, which has denied numerous reports that it is sending thousands of troops to defend Somalia's weak government from attacks by the SICS.
"We are telling you that from today, we are attacking the Ethiopian forces wherever they are inside Somalia and jihad has begun," SICS leader Sheikh Sharif Sheikh Ahmed told the excited crowd.
"I'm calling on everybody that has a gun in his house to take it up and participate in the fight against the Ethiopian invaders," he said.
"Every one of us must swear not to abstain from this call to jihad and the first man to swear is me," said Ahmed, the chairman of the executive arm of the SICS.
Ahmed was once considered a moderate in the movement, which seized Mogadishu from warlords in June after months of fierce fighting and have now expanded their territory to include most of southern and central Somalia.
The response to his call, which came after the SICS supreme leader declared a start to the long-threatened holy war against Ethiopia on Monday, was general enthusiastic, with many young men vowing to fight to the death.
"We are ready, my family of eight is ready, we are all ready to wage jihad to defeat our beloved religion against the invaders," said Ulusow Abdalla, one teenage participant at the rally in southern Mogadishu's Tarbuunka district.
Another teenager, Said Mucaawiya Hashim Abdalla, told AFP that he would join the ranks of the SICS whose forces are currently close to the seat of the transitional government in Baidoa, where defences have been erected.
"If you die, you will be given very important gifts by Allah and if you return alive, you will live honourably," he said. "That is why jihad is very important."
SICS officials said earlier this week that an estimated 3,000 people had enlisted for the holy war since Monday and said on Friday that they had set up an additional seven recruiting stations.
"We have opened seven posts here in Mogadishu for volunteers who are ready to go to jihad," Ahmed Abdulahi, a senior SICS official, told the crowd.
"I'm telling you that every Somali individual is wanted to come to those centres for recruitment then we can start defeating our long-standing enemies and the enemies of Allah," he said.
"The enemy of Allah has come nearer and is intending to attack."
Ethiopia has confirmed it has sent military trainers and advisers to Somalia to help the government but has rejected reports of thousands of uniformed soldiers on Somali territory.
On Tuesday, the SICS claimed to have captured an Ethiopian officer in clashes that killed at least 51 people north of the southern port of Kismayo, but Addis Ababa has not responded to the allegations.
Ethiopia has vowed to protect itself and the Somali government from the "jihadists," whom, together with the transitional government, it accuses of links with Osama bin Laden's al-Qaeda network.
The SICS deny this, insisting their sole aim is to restore stability in Somalia.
The soaring tensions between the SICS and the government and worsening security in south and central Somalia have forced tens of thousands to flee into neighbouring Kenya and added to concerns of widespread conflict.
The deteriorating situation threatens to scupper a planned third round of Arab League-mediated peace talks between the government and the SICS set to begin on October 30 in Khartoum.
Somalia has been without a functioning central administration since 1991 and the government, formed in neighbouring Kenya in 2004, has been wracked by infighting and its inability to assert control over much of the country.

Copyright AFP (Agence France-Presse), 2006