'Til Ungdommen' [To Youth], a song by Norwegian poet Nordahl Grieg that became an unofficial anthem in the aftermath of Utøya

Also known by the words of the first line "Kringsatt av fiender" (Surrounded by enemies), by Norwegian poet Nordahl Grieg. Written in 1936 during the runup to World War II and set to music in 1952 by the Danish composer Otto Mortensen. (Wikipedia) adds, "After the terror attacks of 22 July 2011, it was used in many memorial gatherings and services." The first verse makes clear why so many people found it appropriate in the aftermath of Utøya:

Kringsatt av fiender, gå
inn i din tid!
Under en blodig storm –
vi deg til strid!

In English translation, by Rod Sinclair:

Faced by your enemies
On every hand
Battle is menacing,
Now make your stand

During memorial service __ July 2011 in Oslo Cathedral. Clip from Norwegian National Broadcasting (NRK) of the service in the capital city's cathedral (Domkirken) the Sunday after the terrorist attack. Title says, "King and queen cry during song in cathedral" Original on nrk.no website.

Herborg Kråkevik in concert on Rådhusplassen in Oslo. Clip from NRK coverage 25 July 2011. Kråkevik is a crossover folk/pop singer who recorded the song in 2000, which was rereleased and topped the charts again last year after Utøya. An estimated 200,000 attended the concert in the square in front of Oslo's city hall (Rådhus).

Chords, in Norwegian, at http://nortabs.net/tab/3029/ and on English-language website tabs.ultimate-guitar.com. Song is in A minor (although the copy of the Dansk Skolesangbog I got last year in Copenhagen has it with chords and standard notation in E minor). Below is a very nice finger-style guitar arrangement by YouTube user loooovelyy, 20, of Norway, who says she "looked at the chords on ultimate-guitar and figured out the rest by ear."

Also in Denmark. Expressions of solidarity from across Europe, ranging from a moment of silence at football matches to religious services attended by heads of church and state, were posted to YouTube. Here a segment of TV news coverage of Danish Queen Margrethe, Crown Prince Frederik and Crown Princess Mary as they participate in a memorial service at Denmark's Vor Frue (Our Lady) cathedral in Copenhagen. Video clip of choir singing "Til Ungdommen" begins at 8:34 and lasts 10 or 15 seconds.