Police have launched a murder investigation after Belfast journalist Lyra McKee was shot and killed in gunfire as officers were attacked with petrol bombs and other missiles in Londonderry on Thursday night.

Key points: Petrol bombs were reportedly thrown in the area before Lyra McKee, 29, was killed

Petrol bombs were reportedly thrown in the area before Lyra McKee, 29, was killed Officers administered first aid before taking her to hospital, where she died

Officers administered first aid before taking her to hospital, where she died The unrest came ahead of the Easter weekend, the anniversary of the 1916 Easter Rising

In a statement on the Police Service of Northern Ireland website, Assistant Chief Constable Mark Hamilton said Ms McKee "was murdered during orchestrated violence in Creggan last night".

As the violence unfolded on Thursday evening, Ms McKee posted a photograph upon arrival at the scene with the caption: "Derry tonight. Absolute madness."

A screenshot of a tweet uploaded by Lyra McKee shortly before she attended the scene of the riots. ( Supplied )

According to Mr Hamilton, "a single gunman fired shots in a residential area of the city and as a result wounded McKee".

"Officers quickly administered first aid before transporting her in the back of a Land Rover to hospital. Tragically she died from her injuries," he said.

"At this stage we believe her murder was carried out by a violent dissident republican.

Lyra McKee pictured in 2017. ( Supplied: Facebook )

"This murder demonstrates all too starkly that when terrorists bring violence and guns into the community, members of the public are placed in severe danger."

Mr Hamilton said police searches had been carried out in Creggan before the violence erupted on Thursday night because police believed dissidents "were storing firearms and explosives for a number of planned attacks and these may have been used over the Easter weekend in the city".

He said the searches began just after 9:00pm, when a crowd gathered and "before long upwards of 50 petrol bombs were thrown at officers".

"Coming up to 11:00pm we estimate there were about 100 people in the area including young people and members of the media. It was around this time that the gunman fired a number of shots."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 44 seconds 44 s Petrol bombs have been thrown at Northern Ireland police in Londonderry.

Mr Hamilton appealed to the public for help in finding the gunman and appealed for calm over the Easter weekend, asking that "everyone with influence in the community [work] together to ensure there are no further acts of violence".

Late on Thursday night, police had said they were treating the shooting death of a young woman in Creggan as a terrorist incident.

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'Sick to my stomach tonight'

Journalist Leona O'Neill said she was standing with the woman next to a police Land Rover, and saw her fall to the ground after being hit.

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O'Neill had earlier posted videos of police vehicles being pelted with what she said were dozens of petrol bombs, bricks, bottles and fireworks.

The videos showed two vehicles were left in the middle of the street and set on fire.

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O'Neill later posted more detail about the shooting on Facebook.

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The leaders of Northern Ireland's two largest political parties condemned the killing.

The deputy leader of the republican Sinn Fein party, Michelle O'Neill, said her "heart goes out to the family of the young woman shot dead by so-called dissidents".

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In a statement on the Sinn Fein website, Ms O'Neill described the shooting as "a senseless loss of life".

"Those responsible should listen to the people, they should disband immediately and end their pointless actions against the community which tonight has tragically claimed the life of a young woman," she said.

Mark H Durkan, Social Democratic and Labour Party Member of the Legislative Assembly for Foyle, said he was "heartbroken and angry" by the killing.

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Democratic Unionist Party leader Arlene Foster said the shooting was "heartbreaking news".

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The riots broke out ahead of the Easter weekend, a time when republicans mark the anniversary of the 1916 Easter Rising.

The detonation of a large car bomb outside a courthouse in Londonderry in January highlighted the threat still posed by militant groups opposed to a 1998 peace deal that largely ended three decades of violence in the British-run province.

US House of Representatives Speaker Nancy Pelosi led a congressional delegation to the city earlier on Thursday, as part of a trip to show support for the peace agreement politicians in Washington helped to broker.

The riots in Creggan were reportedly in response to a house search conducted in the area. ( AP: Niall Carson/PA )

ABC/Reuters