Police in Londonderry believe dissident republicans were responsible for two bomb attacks on Thursday night.
The bombs exploded at the tourist centre on Foyle Street and on Strand Road, close to the DHSS office, within 10 minutes of each other.
Homes and businesses in the city were evacuated and no-one was injured.
The PSNI District Commander, Supt Stephen Martin, blamed dissident republican groups for the attacks.
"There's no doubt the principal line of inquiry will be one of the republican groups - probably either the real IRA or Oglaigh na hEireann. I would be surprised if it wasn't one of those," he said.
"They're not very large devices and as I understand the damage to the tourist information centre is not insignificant. There's a lot of window damage and doors have been blown in. There's also internal damage.
"These people are small in number, in terms of support and weaponry. Whilst they are small, they pose a severe threat.
"I think the policing response has been very good. Our commitment is to continue to work really hard."
Londonderry will be UK City of Culture in 2013. Last year dissidents attacked the office used by the organisers of the festivities.
"Our plan for 2013 is multifaceted," said Supt Martin.
"I don't think it would be in any of our interests if we throw a ring of steel around this city for 2013. I don't think there's anything cultural about that."
Police received two warnings about the bombs, one at 19:33 GMT and the other at 19:47 GMT. The first bomb went off on Foyle Street at 20:44 GMT and the second explosion occurred at 20:48 GMT.
BBC Radio Foyle reporter Jenny Witt visited the scene of one of the bomb attacks on Friday.
She said there was still quite a big police cordon in place.
"The tourist information centre right beside the Foyleside Shopping Centre and the entire stretch of road that leads into the carpark is cordoned off," she said.
"That led to huge problems on Thursday night because people had their cars locked into the carpark and couldn't get them out until about 14:45 GMT on Friday afternoon.
"This security operation is now about gathering evidence. It is going to continue for quite some time by the looks of it."
A number of elderly residents had to be moved from a residential home during the police operation.
One woman said she had been told there was a bomb at the bottom of the street.
"We heard one (bomb) go off. It was very loud," she said. "It was getting everybody out of the place where we live. But we all got out safely enough. They sent a bus for us and took us to a hotel."
SDLP Foyle MP Mark Durkan expressed anger at the explosions.
"These attacks on people trying to bring things forward in this city are cowardly, destructive and totally unjustifiable," he said.
"I appeal to those responsible to cease their violent campaign and urge people with information about the perpetrators to pass any information they may have about these incidents onto the police urgently."
Sinn Fein MLA Martina Anderson condemned "the mindless actions of organisations that command miniscule support".
"We now have the opportunity to realise our goals and aspirations through peaceful and democratic methods, through persuasion and force of argument instead of force of arms," Ms Anderson said.
"Whatever organisation planted these bombs needs to come forward and explain how they believe that it has achieved anything other than disruption of older people who have had to be evacuated to a local hotel."
DUP MP Gregory Campbell said the people behind the bombings should realise they would not win.
"They're not going to defeat us if they bomb next week, next year or in 20 years time, they are not going to defeat the people of Londonderry," he said.
"And the UK City of Culture is going to come and we're going to enjoy it. That's going to happen no matter what they do."
Ulster Unionist leader Tom Elliott blamed dissident republicans for the attack.
"This type of fascism was part of our past. It has no place in our future."
Police said on Friday night that all the roads in the city had now reopened.