Somalia's parliament has declared the sacking of the prime minister illegal and passed a confidence vote in him by a huge majority.
President Abdullahi Yusuf sacked Prime Minister Nur Hassan Hussein on Sunday, saying he had failed to bring peace.
But the president said he would comply with any decision by parliament.
Hundreds of people have also demonstrated in the capital Mogadishu in favour of Mr Nur, carrying his portrait through the streets.
African Union Commission head Jean Ping condemned the dismissal and Mr Nur said it was an attempt to derail UN-sponsored peace talks with Islamists.
Somalia has not had a functioning national government since President Mohamed Siad Barre was overthrown in 1991.
President Yusuf's administration only controls parts of Mogadishu and the town of Baidoa after recent advances by Islamist insurgents and is dependent on international aid and Ethiopian military support to function.
'Treason'
BBC East Africa correspondent Peter Greste says President Yusuf made a calculated gamble - that enough parliamentarians would support his move in what now appears to be a colossal battle between rival political forces.
SOMALIA'S WOES
"One hundred and forty-three MPs recognised the existence of the government, 20 rejected it and seven abstained," Speaker Aden Mohamed Nur said after counting the votes during a special parliament session in Baidoa, reports the AFP news agency.
"Therefore the government of Nur Adde [Nur Hassan Hussein] is legitimate," he said.
Correspondents say the prime minister is the one member of the government who could attract international support for the country.
He was chairman of Somalia's Red Crescent Society during many years of conflict before being named prime minister a year ago.
President Yusuf announced the dismissal of his prime minister at a news conference in Baidoa on Sunday.
"The government has been paralysed by corruption, inefficiency and treason," he said.
Later in the day, hardline Islamist insurgent group al-Shabab held a news conference in Mogadishu vowing never to talk to the government or accept any political power-sharing.
Power vacuum
Somalia is in the throes of a serious crisis.
The Ethiopian troops which helped government troops drive Islamist forces from the capital, Mogadishu, two years ago are due to pull out in just over two weeks.
And a small African Union peacekeeping force has indicated it would leave with the Ethiopians unless it gets reinforcements.
Somalia's two leaders had clashed in recent months over attempts to deal with the Islamist-led opposition.
The rift between the two leaders has spread to parliament, where supporters of President Yusuf have previously threatened to pass a vote of no-confidence in the prime minister.
Mr Nur was appointed prime minister in November 2007 to replace Ali Mohamed Ghedi, who had refused to negotiate with armed Islamists and other opposition groups.
Somalia has been racked by fighting between different armed groups for 17 years.
The anarchy has led to a surge in pirate attacks on international shipping off its coast.
Some three million people - around a third of the population - need food aid, aid agencies say.
About one million people have fled their homes - many after fierce fighting in Mogadishu between Islamists and the Ethiopia-backed government forces.
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