Detectives investigating a suspected car bomb in Northern Ireland have arrested two more men.
The men, aged 34 and 42 years, were arrested in the city on Sunday evening.
Their arrests come after two men in their 20s were arrested in the early hours of Sunday morning. All four men were being held in custody on Sunday evening.
An explosion in Londonderry city centre occured at 8pm on Saturday night after a vehicle exploded on Bishop Street.
The Police Service of Northern Ireland said it was fortunate that no-one was killed or injured.
Dramatic CCTV footage of the incident released by police has revealed how a group of people had walked past the car containing the bomb just minutes before the explosion.
Assistant chief constable Mark Hamilton described the bomb as a "crude device" and the attack as "unbelievably reckless".
Mr Hamilton said hundreds of people, including hotel guests and children from a church youth club were evacuated from the area.
Police said their main line of inquiry is that the New IRA militanat group was responsible for the attack.
The Irish nationalist New IRA is one of a small number of militant groups opposed to a 1998 peace deal that largely ended three decades of violence in the British-run province. They have carried out sporadic attacks in recent years.
Assistant Chief Constable Mark Hamilton said he did not see the bombing as an escalation of militant groups' capabilities but a continuation of a threat that has been officially at the "severe" level for 10 years.
Politicians across the United Kingdom and Republic of Ireland have condemned the attack.
