NAIROBI, Kenya, Sept. 25 — Kismayo, one of the largest cities in Somalia, fell to Islamic forces without a shot on Monday — but then the trouble started.

After the city’s warlords fled and hundreds of Islamist fighters poured in, demonstrators took to the streets and hurled stones at the Islamists.

Islamist troops responded with machine guns, opening fire on the demonstrators and killing at least one teenage boy, witnesses said.

“We don’t want the Islamic courts!” the demonstrators yelled, referring to the Council of Islamic Somali Courts, the official name of the Islamic forces.

It was the latest episode of turmoil in a country notorious for it. The Islamist forces took over Mogadishu, Somalia’s capital, in June and have been expanding their reach ever since. The people of Mogadishu have by and large supported them, thankful for the stability they brought after 15 years of anarchy.

But apparently many in Kismayo did not feel the same way. Kismayo is a major port city on the southern Somali coast, not far from Kenya. It was ruled by warlords called the Jubba Valley Alliance, which had vowed to resist the Islamists.

On Sunday night, as hundreds of Islamist fighters in heavily armed vehicles encircled the city, the warlords fled and their militias evaporated behind them.

On Monday morning, the troops rolled into the center of town and were initially greeted with cheers.

“Welcome to Islam,” announced one of the Islamist commanders, Sheik Hassan Turki.

Witnesses reported that a band of fighters then took down a Somali flag and in its place hoisted a black Islamic one. Demonstrations followed, with young men rushing into the streets to burn tires, set up barricades and throw stones.

By nightfall, the situation was calmer, witnesses said, but still edgy.

Adding to the tension were news reports that hundreds of Ethiopian troops had crossed the border into Somalia in what appeared to be a bid to protect the fragile transitional government in Baidoa, a midsize city north of Kismayo.

Islamist leaders in Mogadishu vehemently object to the presence of foreign troops on Somali soil. One reason why they seized Kismayo, they said, was to establish a base to deter any foreign troops from trying to enter Somalia from Kenya.

Annotazioni − Mohammed Ibrahim contributed reporting from Mogadishu, and Abukar Karyare from Baidoa.