A bomb exploded on a shopping street in the centre of Northern Ireland's second city Londonderry on Saturday but no injuries were reported as the area was evacuated after a warning.
The blast happened in a branch of the Spanish-owned Santander Bank one day after Queen Elizabeth ended the first visit by a British monarch to the Republic of Ireland since independence in 1921, a visit opposed by some Irish nationalist groups.
Security forces said they were investigating reports of another device at a nearby branch of mortgage lender Halifax.
"This bomb could clearly have caused death or serious injury," Northern Ireland Justice Minister David Ford said.
An army bomb disposal squad was at the scene but the device exploded before it could be disarmed. Shoppers and staff were evacuated from stores and businesses over a large area of the city centre.
The city is a centre of activity by so-called dissident republicans, Irish nationalist groups opposed to a 1998 peace deal which ended 30 years of wide-scale armed opposition to British rule.
Security had been intense on both sides of the border against an attack during the queen's visit, which passed with relatively little disruption.
Dissident republicans last month claimed responsibility for the murder of a Catholic policeman, who was killed when a bomb exploded under his car.
Mainstream nationalist party Sinn Fein, which is part of a power-sharing government with pro-British rivals, immediately condemned Saturday's attack.