Two men have been arrested in connection with the killing of Northern Irish journalist Lyra McKee during a riot in Londonderry on Thursday, police said, as politicians in the divided region united to condemn the attack.

McKee, an award-winning 29-year-old journalist who was writing a book on the disappearance of young people during decades of violence in Northern Ireland, was shot dead as she watched Irish nationalist youths attack police following a raid.

"Major Investigation Team detectives have arrested two men, aged 18 and 19 under the Terrorism Act, in connection with the murder," the Police Service of Northern Ireland said in a statement.

Northern Ireland's political parties, which are broadly split between Irish nationalists aspiring to unite the British region with Ireland and unionists who want it to remain British, united to condemn the attack.

In a joint statement, six parties said they were "united in rejecting those responsible for this heinous crime."