In a series of edicts, statements and court rulings issued just days after a failed attempt to assassinate the president of the country's transitional government, the Supreme Islamic Council of Somalia (SICS) moved to strengthen its hold on power.
Already in control of the capital, Mogadishu, and much of southern Somalia, the Islamists said they would seize the port of Kismayo and close the border with Kenya to prevent an east African peacekeeping force entering the country to shore up the government's limited authority.
They also sentenced two alleged murderers to be executed in public, arrested a male martial arts coach and six female students, and banned the sale and use of the popular stimulant khat during the upcoming Muslim holy month of Ramadan.
Sheikh Mohamed Nur Duale, a top SICS member in the Lower Juba Valley, said the Islamists had surrounded Kismayo, about 500km south of Mogadishu, and would soon take it from a local armed group.
"We will not attack our Muslim brothers in Kismayo or any other place in the region. Our objective is to defend the country from the enemies of Allah," he said.
"No one should dare stand in front of this holy objective."
Kismayo is currently held by the Juba Valley Alliance, an armed group led by the government's defence minister. But it has been encircled for several days by opposition forces, who are negotiating what they say will be a peaceful handover.
Duale said the Islamists were intent on taking Kismayo because African Union-backed plans for the regional peacekeeping mission called for troops to land there.
Meanwhile, witnesses said the Islamists had boosted their presence along Somalia's remote and largely unpatrolled frontier with Kenya, where the vanguard of the 8,000-strong peacekeeping force from the seven-nation Inter-Governmental Authority on Development is to gather.
One commander from the town of Dhobley, close to the Kenyan border, said: "We will close our border with Kenya before foreign troops can set foot on our soil."
But already crippled by fractious infighting, the administration has been unable to assert control and the Islamists have moved in to fill the power vacuum, sparking fears of a Taliban-style takeover of Somalia.
On Thursday, an Islamic court sentenced two men to be executed in public in Mogadishu on Friday after convicting them of killing a businessman while attempting to steal his mobile phone, officials said.
Sheikh Yusuf Mohamed Siad, the SICS security chief, said: "We are encouraging this practice to stop illegal killings in the country."
In addition, the Islamists ordered a ban on the daytime sale or consumption of khat and the closure of all restaurants beginning Saturday, when Ramadan is likely to start.