Dissident republicans were behind Thursday night's bombs that rocked the centre of Derry, police have said.

The devices exploded at the tourist centre in Foyle Street and on Strand Road close to a benefits office. Families and businesses had to be evacuated during a security operation to clear the areas.

The Police Service of Northern Ireland's district commander for Derry, Superintendent 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 Óglaigh na hÉireann. I would be surprised if it wasn't one of those.

"There is a severe threat in Northern Ireland, which means an incident like this can happen at any time and without warning. We are always prepared for this type of thing. CCTV will be a key line of inquiry."

No one was injured in the explosions, which occurred within 10 minutes of each other. Martin described the attacks as "callous and cowardly".

Unionist and nationalist parties have united in their opposition to Thursday's bombings and the ongoing terror attacks in Derry.

The Real IRA has an active unit in Derry, which has been responsible for a number of attacks on symbolic targets across the city. Two years ago the terror group bombed a branch of the RBS-owned Ulster Bank, and last year admitted responsibility for two bomb attacks on branches of the Santander bank. The Real IRA later told the Guardian in an interview it had targeted banks as part of a new campaign against bankers and the banking system.

It has also repeatedly targeted the offices of the UK City of Culture in Derry. Northern Ireland's second city will become the UK's European City of Culture in 2013. Hardline republicans reject the notion that Derry, with its nationalist majority, is deemed part of the UK because of the award.