This penitential chant hymn is based on a 10th century Mozarabic Litany for the Lenten Season. It is considered to be in the Lydian mode.
See a list of other popular chant themes here.
Videos:
Lorenzo Bonoldi – Intermezzo on Attende Domine – Basilica di San Carlo, Milano
Lorenzo Bonoldi – Toccata, Adagio e Finale on Attende Domine – Basilica di San Carlo, Milano