Islamist fighters in Somalia's capital, Mogadishu, shot at a crowd angered at shortages of the mild stimulant khat, killing one person, say eyewitnesses.
Chewing khat is a popular pastime with many Somali men
Khat vendors were protesting about loss of revenue since a ban on Kenyan flights to Somalia on Monday, that has led to a shortage of imported khat.
The Islamists have subsequently burned two big khat consignments which were flown in from elsewhere this week.
The Islamic courts have tried to outlaw khat since they rose to power in June.
The Union of Islamic Courts (UIC) that now rules the capital says khat encourages immorality.
Most khat, chewed by many Somali men - especially the gunmen who have fought for control of the country for the last 15 years - was flown in from neighbouring Kenya.
Kenya banned all flights to Somalia, citing security fears.
There were six flights a week from Nairobi to Mogadishu and services to three other towns and many more khat flights each day.
The khat protesters were burning tyres and throwing stones before shots were fired.
"We were demonstrating... [when] they opened fire on us," protester Nur Aden Wajishe told AFP news agency.
Other eyewitnesses described seeing a person die of bullet wounds and several others injured in the shooting.
"A 13-year-old boy was killed," resident Ali Suleiman told Reuters news agency.
"I saw an injured man lying on the ground, he was bleeding profusely," he said.
The BBC's Hassan Barise in Mogadishu says a dusk-to-dawn curfew has been imposed on the city.
The consignment of khat burned on Tuesday at an airport 50km south of Mogadishu was estimated to be worth $40,000 and to have originated from Ethiopia, local media reports.
Wednesday's shipment was incinerated because it landed at Mogadishu's main airport where khat imports are not allowed, officials say.
The Islamists have taken control of most of southern Somalia since seizing the capital in June.
Many Mogadishu residents have welcomed their rule as they have brought law and order to the city after years of anarchy.
Khat has not been officially banned in the capital as it has by Islamist hardliners in Kismayo, south of Mogadishu.
But according to the Union of Islamic Courts' website, Kenya's flight ban has seen mosque attendance rise at prayer times.
In some parts of the country, Islamists have been closing public cinemas and, according to some residents, enforcing strict dress codes.
There are fears of a regional conflict starting in Somalia, as Ethiopia backs the weak interim government based in the city of Baidoa and its rival Eritrea is accused of arming the Islamists.