Two teenagers have been arrested over the killing of a journalist shot dead in Northern Ireland, police said on Saturday, after a murder that has triggered international condemnation.

The 18 and 19-year-old men were arrested in Londonderry under anti-terror laws and taken to Belfast for questioning, the Police Service of Northern Ireland said.

Journalist Lyra McKee, 29, was shot in the head late on Thursday in Derry by, police believe, dissident republicans linked to the New IRA paramilitary group as they clashed with police in Northern Ireland's second city.

Tributes to the 29-year-old were led by her partner, Sara Canning, who said McKee's "amazing potential was snuffed out by this single barbaric act".

The leaders of Northern Ireland's main political parties — who have been unable to form a regional government for more than two years — joined together to condemn the killing.

Detectives hunting the gunman had released security camera footage of the unrest on Derry's Creggan estate, hoping that the community would help trace her killers.

The footage showed McKee standing with other reporters by a police vehicle, and the suspected gunman.

Images on social media showed a car and van ablaze and hooded individuals throwing petrol bombs and fireworks at police vehicles.

Police chief Mark Hamilton on Friday said "a single gunman fired shots in a residential area of the city and as a result wounded Ms McKee".

Armed groups

Some officials blamed Thursday's unrest on the "New IRA", a republican paramilitary group opposed to the shift towards non-violent tactics to bring about a united Ireland.

The New IRA (Irish Republican Army) is an amalgam of armed groups violently opposed to Northern Ireland's peace process. It claimed responsibility for parcel bombs sent to London and Glasgow in March.

Catholic priest Father Joseph Gormley, who read the last rites for McKee, told BBC radio: "This rioting was clearly orchestrated. I have no problems saying that: everybody in our community knows that.

"We have a small group of people who want to play political games with our lives."

The IRA sought Northern Ireland's integration into the Republic of Ireland through violence.

The group called a final ceasefire in 1997 and announced an end to its armed campaign in 2005, stating that it would seek to achieve its aims through peaceful political means.

Dissident splinter groups have sought to continue.

The 1998 Good Friday peace deal largely brought an end to three decades of sectarian bloodshed in Northern Ireland between republican and unionist paramilitaries, as well as British armed forces, in a period known as "the Troubles".

There have been concerns that paramilitaries could be seeking to exploit the current political turbulence over Northern Ireland and its border with the Republic of Ireland, caused by Brexit.

Michel Barnier, the EU's chief Brexit negotiator, said the killing was "a reminder of how fragile peace still is in Northern Ireland. We must all work to preserve the achievements of the Good Friday Agreement."

Former US president Bill Clinton tweeted: "Heartbroken by the murder of Lyra McKee and the violence in Derry.

"The challenges in NI today are real-but we cannot let go of the last 21 years of hard-won peace and progress. This tragedy is a reminder of how much everyone has to lose if we do."