A bomb has exploded outside the City of Culture offices in Londonderry.
Police say they are not aware of any injuries and the damage to the building appears to be relatively minor.
A warning with a recognised codeword is understood to have been given at around 22:00 BST - less than an hour before the explosion in Guildhall Square.
Security sources say the attack had all the hallmarks of dissident republicans, who damaged a door of the same building with a pipe bomb in January.
Witnesses to the latest bomb said it was a large blast which left smoke billowing out of the building.
'Extremely loud'Resident Karl Hamilton said the force shook his house across the river and woke most of his children up.
"I had just finished putting my children in bed about 20 minutes beforehand when we suddenly felt the house shake," he told the BBC.
"Windows and doors rattled, accompanied by an extremely loud bang.
Analysis
The latest attack seems to be a statement of intent by dissident republicans.
It appears to be a signal that they intend to try to disrupt the festivities when Derry takes on the City of Culture role in 2013.
Two dissident groups, the Real IRA and Óglaigh na hÉireann, have been active in the area in recent months.
People, politicians and the police have been telling them to stop.
But they are refusing to listen.
"Initially I thought it was fireworks before realising it was the bomb. The younger children still think it was a firework - that way they won't be worried.
"The older ones, however, are immediately broadcasting the news on Facebook."
He added: "I really don't understand why the perpetrators of such action actually think they represent the vast majority of people in our city when most of us just want a simple life."
After receiving the warning, police cleared the immediate area including a number of bars and restaurants and called the Army bomb squad, but the device went off before they arrived.
A large part of the city centre remains cordoned off.
Foyle MP Mark Durkan said: "Thankfully no-one has been injured but those graces are no thanks to those who are behind this attack.
"They are out to destroy and they don't care if they injure or kill when they are at it.
"Derry is a city with many challenges and with many difficulties. But the City of Culture is one of the opportunities we have."
He added: "This callous and dangerous act flies in the face of the efforts made by so many people to improve life here."
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