Sudanese president Omar al-Bashir was ousted by the army on Thursday, brought down by months of anti-government protests against his three decades of iron-fisted rule.
"I announce as minister of defence the toppling of the regime and detaining its chief in a secure place," Defence Minister Awad Ibnouf said in a sombre televised address to the nation.
A transitional military council would replace Bashir for two years, he said, adding that the country's borders and airspace would be shut until further notice.
The veteran leader, who swept to power in a 1989 coup, was one of Africa's longest serving presidents. He is wanted on charges of genocide and war crimes by the International Criminal Court.
Since early morning huge crowds of jubilant Sudanese had begun thronging squares across the centre of Khartoum on Thursday as the army promised an "important announcement".
Chanting "the regime has fallen," thousands poured into the open ground outside army headquarters where defiant protesters have braved tear gas to keep up an unprecedented sit-in now in its sixth day.
The protests, which erupted in December over the government's tripling of the price of bread, were the biggest challenge yet to Bashir's long rule.
The security agency also announced it was freeing all political prisoners.
Army vehicles carrying troops were seen deploying across the centre of Khartoum from early Thursday.
Troops raided the offices of the Islamic Movement, the ideological wing of Bashir's ruling National Congress Party, witnesses told AFP. And martial music was played on state television as soldiers ordered the TV to halt its normal programming.
Outside army headquarters, dozens of joyful protesters climbed on top of landcruisers and armoured vehicles that had been posted to protect them from intervention by other branches of the security forces.
Braving the searing 42 degree Celsius (108 degree Fahrenheit) heat, protesters hugged and kissed soldiers in the crowd.
'Political detainees freed'
Sudan's feared intelligence service said it was freeing all the country's political prisoners, state media reported.
"The National Intelligence and Security Service has announced it is releasing all political detainees across the country," the official SUNA news agency said.
But in the eastern cities of Kasala and Port Sudan, protesters stormed NISS buildings after the releases failed to materialise, witnesses said.
Protesters approached the NISS building in Kasala demanding that officers free their prisoners, a witness told AFP by telephone from the city.
"But NISS officers fired in the air after which protesters stormed the building and looted all the equipment inside," he said.
Protesters chanting slogans against Bashir also stormed an NISS building in Port Sudan, a witness said.
The raids on NISS buildings came despite a call by protest organisers for demonstrators to refrain from attacking government figures or buildings.
"We are calling on our people to control themselves and not to attack anybody or government and private properties," the Alliance for Freedom and Change (AFC), the umbrella group that is spearheading the protest movement, said in a statement.
"Anyone found doing this will be punished by law. Our revolution is peaceful, peaceful, peaceful."
"We had enough of this regime -- 30 years of repression, corruption, rights abuses, it's enough," said one protester at the sit-in.
Demonstrators have spent five nights defiantly camped outside the sprawling headquarters complex, which also houses Bashir's official residence and the defence ministry.
There has been an often festive mood at the sit-in, with protesters singing and dancing to the tunes of revolutionary songs.
The demonstrators have braved repeated volleys of tear gas from members of the NISS since they began camping outside the complex on Saturday, protest organisers say.
Officials say 49 people have died in protest-related violence since demonstrations first erupted in December.
"I hope our revolution will achieve its goal," said Alaa Salah, dubbed the protest movement's "Nubian queen", after a video clip went viral of her conducting chants with demonstrators outside army headquarters.
Earlier this week, the US, Britain and Norway for the first time threw their weight behind the protesters.
"The Sudanese authorities must now respond and deliver a credible plan for this political transition," the countries' Khartoum embassies said in a statement.
'I announce as minister of defence the toppling of the regime and detaining its chief in a secure place,' Defence Minister Awad Ibnouf said in a sombre televised address to the nation.