Sudan's powerful intelligence service announced Thursday it was freeing all the country's political detainees, state media said.
"The National Intelligence and Security Service has announced it is releasing all political detainees across the country," the official SUNA news agency said.
The announcement came as huge crowds of people thronged central Khartoum after the state television said that the army is set to make an "important announcement soon".
But in the eastern cities of Kassala and Port Sudan, protesters stormed NISS buildings after the releases failed to materialise, witnesses said.
Protesters approached the NISS building in Kassala 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 the 30-year rule of President Omar al-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, 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."