Somalia's main broadcasters have been ordered to close, shortly after the interim president set up a new team to end the "chaos" in the capital.
There are several different radio stations in Somalia
Three top Somali radio stations and al-Jazeera TV are affected. They have been ordered to appear before the national security agency.
A policeman was killed and a convoy of government and Ethiopian troops attacked in overnight violence.
The president returned to Mogadishu last week, after Islamists were routed.
"The city is in chaos. It's not safe," he said, as he appointed a mayor, Adde Gabow, and three other officials to run the city.
Somalia has not had an effective national government for 16 years.
President Abdullahi Yusuf was elected at peace talks two years ago but has been powerless until Ethiopian forces helped drive out Islamists.
Meanwhile, there are unconfirmed reports that senior Islamist leaders may have been arrested after crossing the border into Kenya.
Three local radio stations received a letter, signed by Mogadishu security chief Colonel Ahmed Hassan Ali, ordering them to close immediately:
- Shabelle Radio
- Radio HornAfrik
- Voice of the Koran radio.
Correspondents say the radio stations have stopped broadcasting.
Shabelle Media deputy chairman Mohamed Amin told the AFP news agency he was "disappointed" by the measure.
Government spokesman Abduraman Dinari told another local radio station that those affected were "instigating violence", AFP reports.
"We are not undermining the freedom of expression, we are ensuring the security of the Somali people," he said.
The move comes days after the interim parliament - based in Baidoa - authorised the government to impose martial law in Mogadishu.
The policeman was killed when unknown gunmen opened fire on a police station in the north-eastern Hurwa district.
The convoy was heading to the police station when it was ambushed, sparking a 20-minute gunbattle, in which one vehicle was destroyed, witness say.
"I have seen one Ethiopian military vehicle burning after it was hit by an RPG [rocket-propelled grenade]," said Shine Moalim Hussein.
The government is trying to restore order in Mogadishu
The BBC's Mohammed Olad Hassan says that explosions could be heard in many areas of the city overnight.
It is not clear who is behind these and other attacks but the Islamists vowed to launch a guerrilla war, as they fled the Ethiopian advance. Many armed Islamists are thought to have remained in the capital in hiding.
The violence comes as an African Union delegation is in the city to discuss the deployment of peacekeepers.
Ethiopia says it wants its forces to pull out within weeks.
Over the weekend, the regional body, Igad, sent envoys to seven African countries, asking them to contribute to a proposed 8,000-strong peacekeeping force - Rwanda, Tanzania, Mozambique, Angola, Zambia, Tunisia and Algeria.
So far only Uganda has offered troops - 1,500 - although it needs parliamentary approval.
BBC Africa analyst Martin Plaut says African leaders are concerned about becoming bogged down in a quagmire.
The latest attacks followed an operation by government and Ethiopian troops to disarm Mogadishu residents.
After years of conflict and lawlessness, many Somalia possess their own weapons.
Hassan Mohamoud said troops entered his house early on Sunday and seized his Kalashnikov gun.
"I bought the gun about 10 years ago in order to safeguard myself and my family," he said.
"But now I worry about whether the government will take responsibility for our safety."